1) Will there be limitations on the length of user-built routes, amount of rolling stock, etc. or is that entirely dependent on the user's system? 2) Is there a danger of updates to the editor making our work to that point incompatible, in the way that the game's updates to UE 4.23 and 4.26 caused big problems?
What makes you say that? TSC has broken user content (and even third-party content published by Dovetail) plenty of times.
Because I don't think TSW is moving off of 4.26 any time soon. If TSW is moving to UE5 in the future, then DTG will certainly have a way to convert UE4 content to UE5. They wouldn't abandon their content library.