Alright, I think I have an actual bug this time. In the attached savegame I just started using the Skoda 19 tr on the Eastern and Western Loops. On both I have used choose signals near a couple cities. Whenever the vehicles come to the signals they stop, even when the station is empty and it's their destination. Mac OSX, 111.2.1, noticed first on r5435 loaded again on r5531.
https://docs.google.com/open?id=0B4x-6whHppW7cGZveGNPemdUMWlyY3VzdU5uSEttUQ (https://docs.google.com/open?id=0B4x-6whHppW7cGZveGNPemdUMWlyY3VzdU5uSEttUQ)
These halts are too short. Make them 2 tiles long, then everything will work.
In your case, the choose signal blocks the Skoda trolleybuses since they require 2 tile long stations but there is none.
Something has to be done implementation-wise, too. As there is no error description such that the user can figure out, why there is such a blocking. Maybe make the choose-signal not blocking if there are not suitable halts found ?
If nothing suitable is found, the convoi should go to the stop in the schedule. I.e. find_route fails, and then the original route will be kept. I will have a look at it.
EDIT: solved. YOu will now also get a warning, that the original stop is too short for choosing.