Since the last patch you cannot leave the drivers seat of a locomotive parked on the turntable. Also getting message that the turntable is not aligned. Reproducible!
I have this problem too. As do a bunch of others, see here: http://steamcommunity.com/app/530070/discussions/1/1495615865219521832/
Normally, the turntable house has its own green / red signals on the roof, signifying whether it's safe to proceed onto the turntable.