If you want to return to free roam and pick another service after driving the 66 on GWE you'll have to be quick. The AI driver takes over and applies power as soon as you finish the service giving no time (like it used to) to get out of the locomotive as once it gets to a certain speed, you don't have the option to climb down the stairs and it isn't a loco you can jump out of. This leaves you trapped and eventually, kicked from the route as you disappear down a portal.
They push you out of the seat because the next service is not a player service and is likely going into a portal. The behaviour changed with the Rush Hour update so you don't see 'alight here' services anymore. Ideally the train should only move once the player is not within the loco environment.