Of the common GPS map apps, Google Maps seems to have the most detailed railroad information. It will typically show sidings and yard track layout.
The drawback is that you have to have a pretty good idea of just where you want to go before increasing the magnification down to where railroads and turkey trails appear on the screen.
I prefer DeLorme Gazeteers for finding where to go. Google Maps can be helpful in pinpointing a train watching or photography location. It's easy to switch back and forth between an aerial photomap and a road map on Google Maps.