Tutorial Cab Car - Changing Ends Train Won't Move After Stopping

With this tutorial, after I have stopped the train I sometimes cannot get it moving again. At another time I was able to set the trainbrakes from B2 to Running, and then I could advance the train by setting the throttle to say 6 or 11 %.
But most times when I set the trainbrake ro Running and apply some throttle, the train won't start moving again. Increasing the throttle even to 60% has no effect. When I try to set the train brake to Full Release, after I let go of the handle it moves back to Running.

Which more experienced driver can help me find out what is wrong?