If you go onto Google Maps and use the My Maps function you can mark routes and spots and add descriptions super easily. I have used this function to map out a few rails in my area (London, Ontario). Its pretty sick because you can scope a bunch of them with street view and keep track of the locations of rails you spot.
I was thinking if a bunch of people on here made maps of their area and posted them in this thread we could create a huge database of urban rails. So you could maybe discover some new ones in your area or if you go travel you could have a guide for a few spots to start.
Anyway if you make/have a map post it here ill start.