The back of the consist goes to 0 after every change unless its coupler was
set in the editor; a car that gets a neighbour loses that mark, so it is
freed again once it is last again.
The simulator hangs end signals only on a last vehicle with coupler 0; every
new car came with 3 and the pass that couples neighbours never touched the
last one, nor did taking a car off the end. The coupler editor says so for
the last vehicle.
