Not a ridiculous idea at all!
The key thing is the data and we don't have access to that so sadly as it stands we won't be tracking those!!
Would be nice though!
You may already be aware of Matthew Somerville's 'Vaguely Live Map of Trains in the UK' http://traintimes.org.uk/map/
Just select the Main Line Station from the drop down list.
Train Finder would be a good idea, but with the National Rail application and even the mobile site of National Rail indicates whether a train is at a station or is on its way to another. There is a vertical line with stops marked on them and a blue dot represents the train on the vertical line. Granted it isn't accurate geographically, this is where the Train Finder application can come in. I live on the Southeastern Railway territory and used to work in train planning for a time. The Bombardier (Class 375/376) stock, the Hitachi stock (Class 395) and the Metro Cammel stock (Class 4652/4659) have GPS built in, but only for Passenger Information notifications from the speakers and scrolling screens.
However, there is already a software which tracks trains in real-time on the tracks as they pass signals. The software is called P2 and is not available publicly in any way unfortunately. I have tried copying the files from the server while at work but to no avail when I tried them at home. When clicking on a certain unit on the tracks, information about its class, carriage numbers etc. appear. All stock including freights are viewable. Green stock represents on time, yellow represents 5 minutes or more late while red indicates very late. More information on the software can be found here: http://en.wikipedia.org/wiki/P2_Train_Map
The request for Bus Finder will be backed up by me. In the south east, newer buses have GPS built-in. On Arriva's Fastrack service, maps are viewable on public screens to show passengers waiting at the bus stop where the buses are on the roads. If there is a way to acquire this same or similar software onto mobile phone and tablet platforms, I'm sure customers would buy the applications. I already know a few who would love to have a bus finder application.