Ok i'll send you a mesh if i'm late just lay some rails for me (I have no idea how to make the locomotive turn exactly along the rails, so the rails must be straight without turns)
Don't worry about that, I'll manage through .dll magic ? I know very little about trains, but i'd say the axles are articulated so that they can take turns.
It would be pretty hard to simulate a full rollout from the vehicle integration building, because in Baikonur the distances are huge. Also the train makes stops in different areas for fueling, the ILS manual says there's a specific fueling area for the Briz upper stage. Propellant used for Proton and Briz is very toxic but also stable at ambient temperatures.
Right now my best bet would be to manage all those meshes (rocket stages, fairing, train...) as a big animation of the launchpad. It is a bit tedious from a coding standpoint (because we need reference points and vectors for everything), but there are few limits to what can be done, and there would be no problem with time warp. In theory, provided we can get the vectors and points, a 50km rollout animation is possible.
In theory. Then, when the animation is complete (rocket standing on pad), we spawn the whole rocket stack and dock it in the correct order (the launchpad controls all of this). There will probably be a noticeable "lag" at this point, but that's the price to pay. Simulating the train as a physical vehicle rolling on the ground would lead to many problems, including time warp.
I'll start small making the train arrive and erect the rocket, once that done we can explore how far we can push the thing ?