One of the most frequently asked questions from new players of the game include, "What are the best mods?" and "What do I need to download?" To that, there are two blunt answers: there are no "best" mods, and you don't "need" to download anything. And, for that, you deserve a more nuanced explanation. After this, I'll discuss the variety of content that is available.

OpenTTD is an open-source clone of the video game Transport Tycoon Deluxe released in 1994. Over the next decade the game was left with unaddressed bugs, and eventually the installation drivers no longer worked for newer operating systems. OpenTTD exists primarily to allow players of the original game to continue to play the game. It is 100% backwards compatible with all previous saved games, scenarios, graphics, sound and music. If you owned the original installation media, you can copy it over and enjoy the game just as you did a quarter of a century ago. Obtaining the original media without piracy is nearly impossible, however. For that, the game's community developed its own open-source versions of the graphics, sound and music.

The game is continually updated and improved upon by its developers, and aims for, at the very least, a major version release every year. The developers also release major bug fixes, as needed, throughout the year. They also provide a continuous near-daily update for bug fixes, edge cases and minor enhancements, but these are not provided for via in-game downloads or updates. If you need an immediate bug fix that has been corrected, you'll need to download and install this separately (and, if you wish, you can install separate copies of the game). Advanced users can download the source code and apply bug fixes and other patches directly to the source code and compile, but most users do not have a coding/development environment set up - it's complicated and messy. OpenTTD's developers are quite possibly one of the most dedicated groups of developers for any game, and they work very hard to build and release a quality product that has stood for sixteen years.

As it exists, OpenTTD itself is a nearly perfect game. It contains all of the content needed to successfully play the original game, as it was designed, from start to finish. It has a very well-balanced gameplay that keeps players engaged and invested, and players find themselves coming back again and again. A single game can last for as long as you want it to, and every new start is unique. Players can make further changes in the game environment to increase their challenges as they become more experienced. It is for these reasons that I say that you do not need to download content or mods. It is an entire package of complete and pure entertainment.

Because of OpenTTD's goal of being a 100% faithful clone of Transport Tycoon Deluxe, it will never change its base features. The base game will never add new vehicles, change industries, or add any features which will break backwards compatibility with the original game, saved games or scenarios. But this is not where OpenTTD ends. The developers recognized that players will want more, and that what is provided may not be enough. They introduced a system for adding new content to the game which allows it to grow and expand in new ways without breaking that backwards compatibility. While that may frustrate some players that parts they feel should be essential will never be added to the base game for distribution, it does allow players to download what they like and create their own unique experiences. None of this content fixes anything that is inherently perceived as broken. Even more importantly, none of it can truly be said to be "better" or "must-have". It entirely depends on your opinions and feelings. One mod may be viewed as the best thing ever by some, while others will find it frustrating and confusing to use. Still, there are some content which are clearly more popular than others. I will not engage in that discussion here, I will leave this for the reader to discover, evaluate, and make up their own mind.

And, there is a lot of it. By my own count, there are well over 2000 pieces of content that can be downloaded, either through the game directly, or found in the wild of the Internet and available to be installed separately. You can only use a small portion of this at any given time due to limitations of the game, but you can just about mix and match any of it (with a few notable exceptions), and for the most part it all works together without any conflict (aside for possibly graphical inconsistencies or glitches), even though almost none of it is inherently designed to do so. It's almost like magic, except it's really due to the fact that the developers have implemented a very strict, structured code environment that helps to prevent the player's experience from becoming corrupted.

OpenTTD's downloadable content and mods come in several different types. And with that comes my first bit of advice - don't download any content until after you have mastered the basic game. Play the game originally as intended several times until you understand the game mechanics, goals and objectives, how to score (if you're playing to score - OpenTTD is simultaneously a competitive game and a nearly open sandbox game), etc. Once you get a good feel for the game, adding content will be an enhancement. If you add content before you understand the basics of the game, it might be a frustrating experience.

NewGRFs (short for New Graphical Resource Files) are the bulk of the available mods. These are broadly broken down into these groups:

• Vehicles: trains, aircraft, watercraft, road vehicles, trams/streetcars;
  
• Industry: special buildings that accept and produce cargo, mail and passengers;
  
• Infrastructure: railroad stations, train rails, rail waypoints, passenger bus stations, cargo lorry stations/truck stops, road surfaces, tram/streetcar rails, bridges, aqueducts, tunnels, airports, water docks, vehicle depots;
  
• Landscape: base tiles that represent grass, snow, sand, rock, water, brushland, and trees;
  
• Town/City Buildings;
  
• Town Names;
  
• Objects: non-functional eyecandy;
  
• Basecost Mods: non-graphical mods which modify costs.

Essentially these mods not only change graphics, but can also modify the costs of things in the game. In the game you are limited to loading a total of 64 NewGRFs. Do this at your own peril. The more content you add, the more complex it becomes to manage it all and you can easily become confused or overwhelmed. My advice is to start simple.

Vehicles have independent variables (speed, power, tractive effort, cargo capacity, aging) and are often not balanced against each other. It's typically advised to only activate ONE type of vehicle set in order to get a consistent experience, but it's a free world., and mixing them up is mostly harmless. But many of these mods can include hundreds of vehicles, and the management list can become quite unwieldy. Total number of vehicle types is capped at around 65,000 (I've never seen anything get close to this), and the base game setting is a max of 5000 trains, 500 road/tram vehicles, 500 aircraft and 500 ships in any active game. This can be changed in-game as needed, but the more vehicles you have moving around, the slower the game may become.

The same can be said for infrastructure. For bridges you're limited to a total of 15 types. The game includes 12 types combined for railroads and road/trams. Bridges can come as individual content or bundled in groups, and can mostly mix-and-match. RailTypes can have up to 64 types installed, and can include various types of non-electric, electric, standard/narrow/broad gauge, monorails, maglevs, and "universal rails", and can include limits on speed and the types of vehicles that run on them. RoadType and TramType (sometimes called NRT/New RoadTypes and TramTypes, or RATT) can have a combined 64 types between them. These typically include basic asphalt roads, unimproved roads, improved surfaces and other upgrades, as well as electrified or non-electrified tram rail and overhead electric for trolleybus/trucks, and can also have speed limits and vehicle limitations. RoadTpe and TramTypes are usually bundled together in one download, and can mostly mix-and-match (but many will have overlapping types of surfaces, which can become confusing). Stations and waypoints are limited to around 65,000, and I don't think that many actually exist in all sets combined, but you can build as many of them as you want in the game.

Industries also have independent variables. Also, as a fairly hard rule, do NOT activate more than one industry mod unless you explicitly know that it's designed to work with other industry mods. One set in particular (ECS Vectors) is broken up into several downloads, for example. There are also a few independent industry mods designed to enhance other industry mods. By and large, though, most industry mods are designed to explicitly disable themselves if a conflicting industry set is also detected as activated. You may not receive a warning if this happens unless you follow-up and check the list of active NewGRFs when you start a game. Best to do your research first, but the worst case scenario is that if this happens, just abandon your game, remove the offending NewGRF from the settings, and start a new game. Also, if you install a NewGRF industry set, you WILL need to install NewGRF vehicle sets of all types in order to transport their cargoes. Almost every vehicle set is designed to transport cargoes from any industry set, so you very rarely need specific sets - if you do, the documentation in the industry set will tell you what sets to use.

Town/city building can be mixed-and-matched, but this is one area where graphical design can be wildly different and inconsistent. Some of it can look quite awful when thrown together. Some sets try to emulate the original Transport Tycoon style, some try to emulate the OpenGFX style (the open source graphics provided by OpenTTD), and many do their own thing, from being simple to being quite ornate or busy. Most sets will disable the TTD/OpenTTD graphics, and some will let you use them simultaneously if they have a parameter to enable it. You'll probably be happier sticking to one. Town/city buildings are NOT buildable by players. The cities in the game select the types of buildings to build. Buildings may or may not provide or accept passengers/mail/cargo. It represents part of the unique challenges of trying to transport them and providing what cities need.

Objects are what players use to get around that limitation. The downside of objects is that they do NOT provide any passengers, mail or cargo. They are purely eyecandy that exist in order for players to create a more pleasing aesthetic and environment for themselves. These can be buildings, parks, water, walkways, farm fields, vehicles, statues... there are no limits to what an object can be or how it can be used. If you've played SimCity or Roller Coaster Tycoon, you'll love these. And there are vast numbers of sets dedicated to objects. There's an in-game limitation of around 65,000 objects. Good luck filling them up!

Town/city name mods can only have one installed and activated at a time. These are popular if you want to include names of towns in particular regions/countries or based on themes. A bit of caution is warranted here. OpenTTD can generate thousands upon thousands of towns, but many name mods are only capable of generating a limited number of names. If you run out of town names, you'll probably get an error and the game may fail to start. Do you research, and if warranted limit the number of towns an OpenTTD game can generate.

Some more content for you to download includes:

• AIs: they could be your worst enemy, or your best friends;
  
• Scenarios: you have places to go...
  
• GameScripts: ... and things to do!
  
• Heightmaps: explore the world outside, or create your own;
  
• Base Graphics, Base Sounds, Base Music: modify foundational parts of the game.
  
• Patches: the scariest, most complex of content.

AIs are also available for download, and while most of them exist to offer you in-game competition in single player games, some of them can also be cooperative working in other companies to take care of things you don't want to manage (for example, pretend you have no interest at all in transporting mail. Let the AI do it!). By default they are not included as part of the base game. Some of these AIs can be quite competitive, sometimes ruthlessly. However, they are subject to all of the same limitations and enhancements as human players, so they can't cheat to win, they can only play the game better (sometimes faster) than you. You cannot load an AI with your own company, and you can only load one AI per company, up to 14 companies. One caution, though. When OpenTTD introduced the New RoadTypes and TramsTypes feature last year, it broke nearly all of the existing AIs that build roads and road vehicles. For right now it's advised that if you want to use an AI, do not install a NRT-enabled road set, stick to the base game roads.

Scenarios are, essentially, a type of saved game created around a theme , and possibly self-directed goals. Scenarios will typically require the additional download of NewGRFs, AIs or GameScripts in order to play them. If you have missing content, the game will offer a way for you to try to find them and download them. These can be quite fun, but a major warning here is that many scenarios (especially the majority of them, which aren't available from the in-game content download service) were created at a time when users were expected to provide their own NewGRFs - a feature disabled over a decade ago because it was just too buggy. Most of these have not been kept up-to-date by authors, so don't be disappointed if you can't even locate the NewGRFs required. My suggestion is to utilize the official OpenTTD forums on tt-forums.net for assistance. That is where most of these scenarios were initially developed and distributed, and the community can typically help you find what you need.

GameScripts offer some advanced features and goals for games. They typically come in two different styles. Citybuilder scripts tend to focus on giving players goals of delivering certain types and quantities of cargoes, mail and passengers in order for cities to grow, so you can then have more of those things to transport out of and between other cities. Goal scripts typically provide softer objectives that are not explicit to city building, but keep you busy and engaged. GameScripts can provide a wide variety of enhancements, so be sure to carefully read up on documentation before you install one. You can only activate one GameScript at a time.

Heightmaps are PNG or BMP image files which can be imported into the game to re-create ground elevation. This is popular for re-creating real-world geographical places or to create unique, challenging grounds aside from the nearly random landscape that OpenTTD can generate. They can not only be renderings of real-world places obtained from government databases, they can literally be any image. Ever wanted to play the game where the landscape looks like a corporate logo? Or your mom's face? You can do that! Of course, not every image imported is going to result in a landscape that you like or even feel is worth playing. That is because for optimal usage, the image has to be a 256-bit greyscale image . OpenTTD does include a scenario editor and some landscaping tools to help you create a heighmap, but there are much better tools out there which can help you create your own or download heighmaps from government databases, or even other games.

Base Graphics are the foundational graphics of the games - landscape, buildings, the base vehicles and other assets, and the GUI. The game can utilize the original Transport Tycoon Deluxe base graphics, it offers download of its own OpenGFX base graphics, or you have third party sets. There's not a lot of them as they contain tens of thousands of sprites and take quite a lot of time to assemble and maintain, but there is some variety. You can only activate one at a time, but a fun feature is that you can save a game, change the base graphics in the main settings, and then reload the game and have a new looking environment. Of course, it's almost entirely likely that none of your NewGRFs will match, but yes you can do it.

Base Sounds are all the sounds of the basic assets you hear in the game - the click of buttons, the toot of a train whistle, etc. Of these you can have original TTD sounds, OpenSFX sounds, or the budding AltSFX set. You can use only one, and as of right now that's probably going to be OpenSFX.

Then there's Base Music. Transport Tycoon Deluxe included a jukebox MIDI music player and its own music. You can only load one base music set at a time, and there's several themed sets you can download and try. You cannot play your own music files, and the player ONLY plays MIDI music in the general MIDI format. I, personally, created several of these sets, and I had a hand in helping to create the OpenMSX set to replace the original TTD music. Please, feel free to try them out... although to be fair, most players simply don't appreciate MIDI music or would rather listen to their own, so they just turn off the jukebox player and disrespect all my work. ;)

And, now, for the very last of all the mods... are patches. Patches are something you are probably never, ever going to mess with. Patches are how you modify the OpenTTD source code to make changes to the core game. They can be bug fixes, or they can add or remove features from the game. They will never be available from the in-game content download service, nor can they ever be. To use them you need to be somewhat proficient in understanding the C++ coding language. You'll need to have a coding/developer environment set up on your computer. You'll need to understand how to fix things if a patch breaks something. You'll have to learn how to compile your own code into an executable binary - the launchable, playable game itself. And if you do this, your copy of the game will probably not be compatible with anybody else's copy of the game, nor will any saved games or scenarios created with your copy be playable on anybody else's copy.

Yet, they are still accessible to players who are willing to learn. Patches are the way OpenTTD developers fix bugs and add features. Independent developers use this as a way to experiment with their own ideas and eventually submit them to the OpenTTD developers for review, critique and possibly inclusion in future versions of the game. As part of this process, developers will release their source code to be compiled or will release compiled binaries for popular platforms so that players can test features themselves and provide feedback. If a binary contains multiple patches, especially if compiled from a variety of developers, it will be referred to as a patch pack. There are a couple of very popular patch packs available, but there won't be available from OpenTTD directly and can't be downloaded from their content service. While they may be popular, keep in mind that due to their experimental nature they'll have more bugs than the base OpenTTD game, This also means, again, they're not compatible with the base OpenTTD client, but because there are many players who use them, you'll find them in use on multiplayer servers. As mentioned previously, you can install multiple versions of OpenTTD, and this includes the patch packs.

So now you're at the end of this very long document. After all is said and done... how do you install these mods? There are two ways:

1. Download directly from the in-game content download service, also called BaNaNaS (an acronym for Base graphics And NewGRFs And NoAIs And Scenarios), also sometimes colloquially called the "Fruit Store". Most of the most up-to-date content is uploaded here by their developers to make it easy to download and install content. Click, download, done. Once complete, go back out to the Main Menu, and either go into NewGRF Settings, or Game Settings (for non-NewGRF content), add/remove/modify what's need, exit out to the Main Menu, and start your new game.

2. Self install. Most of OpenTTD's available content is actually not available from inside the game. That's because much of this content was developed long ago before this system was even available. With that said, a lot of that content is not updated - so some of it may not even work with current versions of OpenTTD, and in some cases other developers have created their own inspired versions of it. Some of it may contain copyrighted content that makes it ineligible to be included in the content download service. Much of it doesn't include a proper distribution license (or any license at all), also disqualifying it for inclusions. Still, for historical purposes you will always be able to download and install your own copies (especially if you ever decide to make your own, but that's a whole 'nother ball of wax). For more information on this, consult the OpenTTD documentation. With that said, at this time (hours before the official Steam launch) I have no idea if that option will be available for Steam users... but I'm assuming it is. If I learn otherwise I'll update this part more definitely.

I maintain an unofficial, non-exhaustive list of NewGRFs apart from the official database, which links to much of this content, at https://wiki.openttd.org/en/Community/NewGRF/, and also links to other OpenTTD communities which also has their own unique content. It can be a fun bit of treasure hunting digging through nearly 20 years of forums, web pages and other content to find unique gems that you may not otherwise see. And the best part, is that it's all free, all created by members of their communities.

Finally, another popular question that gets asked: "How do I add or remove mods from my current game?" The answer is that while you can technically do it, it's disabled by default and VERY MUCH NOT ADVISED to do so. For that reason I will not tell you how it is done, but you can search for it online or ask around.... BUT DO NOT DO IT. The reason why is because modifying any active game, saved game or even a scenario will very likely break it. It may or may not be immediately noticeable (broken graphics, broken sounds, industries producing wrong cargo, corrupted cargo, no cargo at all, etc. etc. etc.), and you may not even experience it until the late stages of your game. Some people may tell you that it's okay if you're only adding new stuff and not removing, or it's okay to remove as long as you remove all the assets from within the active game first. This is not universally true or provable. The reason the developers disabled this feature by default is because players actively adding and removing mods in the middle of a game resulted in bug reports submitted to devs that they either couldn't track down or fix because of the open development nature of the mod environment, plus it's not their responsibility if somebody's mod screws up the game. Don't look for trouble where it's not wanted. It is most advisable that if you wish to make a change, stop playing your current game, and instead start a new game using the mods you want to use. With that said... breaking the game that you've saved and played on for three years of your life, literally thousands of hours of play, isn't the worst thing in the world (yes, I'm speaking from personal experience here). Just be advised that if you play with fire, you occasionally get burned. Asking for help from the devs will be pointless, and likely nobody else is going to have the experience to debug your game to try to determine if it can be saved. If you break it, accept your loss and move forward.