Just incase you not got any further with this. I believe I had this issue and what I did was started the whole train again (turned if off and back on the train that is) Also is this after you change front to back, if so did you set the brakes as the game says.
The game doesn’t tell you how to do a cold start. It should. Instead you have to hunt the internet or YouTube to find out.
Hmm, tried this this afternoon and only managed to get the train moving after throttling upto notch 8! It was then stuck at 14mph and wouldn't speed up so I'm guessing I hadn't fully released a brake or something on the other train? Didn't fancy 32 miles at that speed lol so I'll try again tonight
Tried again tonight and it worked fine this time apart from not letting me change the reverser right at the end to finish the mission lol It worked in the end after messing about selecting other things