Hi,
Seems like our bug friend is quite persistent.
The scenario will happen if you wait about a month near Rome station in the Nor-Italy save, you can actually see the blocking build up.
http://simutrans-germany.com/files/upload/dsa.jpg (http://simutrans-germany.com/files/upload/dsa.jpg)
I think that I have managed to fix this on my 10.x branch...