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SketchUp should be viable.
If you want to make some assets, you can grab Blender for free, but you'll also need a graphics program like Photoshop that you're comfortable with for the textures.
cslmodding.info is a great resource.
As for Sydney trains specifically, NSW Transport's website is great in that they actually have diagrams of most of the trains, that you can load in to Blender as a background image, size up and then basically trace the profile to get the proportions right. Then I used just google images and my drone to get reference photos of the rest.