Nice project idea: collect gps data from cell-phone users that are on a train, and based on this data (lots of users moving at the same speed across known tracks), get an overview of where trains are located.
http://www.apps-bahn.de/bin/livemap/query-livemap.exe/dn?L=v...
Disclaimer: this is NOT realtime GNSS data. New project coming in that direction soon.