I am new to Run8

When I have been playing for several minutes my cap entering keys stop working. At first it works by pressing CTRL+F11 and moving to the end of the train by pressing CTRL+F12. But minutes later, when I move out of the locomotive (F12) I wont be able to enter any locomotives again.

Its still possible to mark a locomotive, but entering the cap is not possible.
Anyone with knowledge of this???