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/
Here's a web service for Helsinki tram GPS data: http://transport.wspgroup.fi/hklkartta/
For some reason it's ridiculously hard to find as it's not linked to from the pages you usually use to find transportation.
And the estimated locations of dutch trains: http://kubus.mailspool.nl/spoorkaart/
Close the dialog box that opens, it's a search for trains or stations on the map.