There seems to be a remnant of a previous bug -- where convoys would not cancel their reservations when the track had changed -- in 11.9. See the following:
https://dl.dropboxusercontent.com/u/61716/reservationcancellation.rar
The next northbound convoy will have to change its route after it gets across the second bridge. But when it does, it leaves behind one square of reservation on the bridge, which will remain and cause a blockade.
Thank you for the report - found and fixed on the 11.x branch.
Thank you!