Thanks, glad you're enjoying it.
I will definitely be adding workshop support for tracks you create, and then after that I am hoping to get custom models working too.
Multiple trains will likely happen, multiplayer I'm not sure about yet. That would definitely need to wait until the rest of the core game is done, and even then it isn't for sure.
Color options will definitely happen.
Automatic mode will likely be part of the multi-train aspect as I'll need to figure out how players can control more than one train, it isn't too feasible with the current system.
Yep, lots more objects of all types are planned. I already have a couple of models like that ready (a tractor for example) which will be releasing soon.