So I've gone into service mode and learned from the 'maintenance manouvre' scenario how to uncouple a unit and now I've done a 2 car unit from Hempstead to New York Penn and upon arriving at the first station; Floral Park its popped up in read saying 'Unexpected Vehicle Formation' - anyone done something similiar etc?
Have you started a service and changed the number of vehicles? Made a longer train out of 2 or something?
Options->Target Marker: On. So you can watch the right point where you have to uncoupled the train. This message happen when your train have too short or too long composition.
If you change a pre-defined Service in Service Mode, couple additional cars on it or uncouple some, the game will get messed up. If you make the train longer out of 2 trains, you will face red signals on the main line. If you make it smaller it gives you that message and might messes up some other things down the line. The Service Mode is designed to be played as it is and messing around with it will cause problems like that.