Thus far, I have been able to confirm that this works on the Cathcart Circle. It may work on other routes. I will play around a bit, and will update the guide accordingly.

Guarding in the UK is a role fulfilled by a guard. It consists of managing passengers, making announcements, filling out paperwork, observing the platform and making sure it is clear, communicating with the driver, managing emergency procedures and, most importantly for our purposes, opening and closing the doors.

Dovetail has recently begun adding guard scenarios to their routes, such as the one on the Glossop Line. However, these are scripted, and can only be performed once. However, the Cathcart Circle line has something of an easter egg: you can guard AI driven services yourself! Naturally, you are limited to opening and closing the doors, but with a bit of roleplay, you absolutely can do the rest...if you don't mind shouting at your PC like a nutter.

• Spawn on foot close to the time a service will depart.
  
• Climb aboard the service. In the lead cab, insert the guard's key, and disable the door control fuse.
  
• Go to the trailing end, insert the guard's key there too, but do not disable the door control fuse.
  
• Guard the train using the rear two guard panels. The front panels do not work. This is how it is done in the real world, too. The guard is at the back of the train. Use the "Open left/right doors" buttons. The "local door control" opens the cab door and nothing else.
  
• Every time the train stops, wait at least 30 seconds before closing the doors. I usually wait 40, just to be on the safe side.
  
• Give two dings with the signal bell to let the driver know that he is cleared to depart.

Sadly, there is no way to know when the vehicle is full and ready to depart, hence the timing. You can technically do this by starting the service and waiting for the AI takeover prompt, and then letting it do so, but I have found this method to be buggy as the prompt is slow to appear and what you may not do for it to appear is rather nebulous. This is the safer, easier method...if you're alright with sacrificing the passenger load indicator.

P.S. I always turn on the AI driver's safety systems too, just for a laugh. He can manage it, and I like torturing my virtual train people with some extra legwork.

Always be mindful of the last passenger stop for the train. What I usually do is remove the guard keys from both cabs and pop the door control for the driver back in after the final loading procedure is complete. That way, you can disembark in a timely manner and let the AI mannequin man-woman-person-thing take the train to the depot.

And from there on out, the world is your oyster. Or, well, you know, twenty odd square miles of Scotland.

Could be worse.