There are sites that do this for traffic and such already, http://commuterfeed.com/ http://commuterfeed.com/. Also if you wanted to do this for the subway you

There are sites that do this for traffic and such already, http://commuterfeed.com/ http://commuterfeed.com/. Also if you wanted to do this for the subway you could just create a hashtag for your rail system. Then when ever they see a train tweet #<hashtag> train just left <station>. This would only require getting the community behind you since the infrastructure is in place. You could then use Summize to track the hashtag and populate a website so not twitter users could use it.