Why won't my train go to the depot for regular maintenance?
Shamelessly copying from Jon:
According to the wiki, the "Two-Way" path signal triggers a penalty in the pathfinder algorithm for trains passing through it on it's back side. I'd venture a guess that when a train is trying to find a maintenance depot, there may be a bug (or feature) where the pathfinder rules out that depot due to the path-finding penalty.