Actual GPS data from trains in Finland: http://www.vr.fi/fi/index/aikataulut/junatkartalla.html (finnish only) Close the dialog box that opens, it's a search for trains or stations on the map.
I did a somewhat similar mobile app for Maemo using the open data we have of Helsinki trams: http://bergie.iki.fi/blog/buscatcher-never_miss_another_tram... Some relevant public transportation data sources for Helsinki: http://www.hri.fi/en/data/hsl-reittiopas-api/
For some reason it's ridiculously hard to find as it's not linked to from the pages you usually use to find transportation.