The good news is that it is coming.
However I can't actually find the real-time open data or API for trains. I am more than happy to be pointed in the right direction.
What I could find:
UK Cabinet Office - Further Detail on Open Data Measures in the Autumn Statement 2011 (PDF)
There is a text extract of the train and bus section available at http://travelinedata.org.uk/opendata.htm
Here are some highlights:
Train and bus data - The Government will work with the transport industry to make available by April 2012 timetable and real-time train and bus information to support the development of innovative applications to improve passenger journeys.
o Real-time running data from Network Rail
...DfT will also work with Traveline to release the following data relating to buses:o Traveline National Dataset on a weekly basis (Great Britain buses); ando Next Buses API of planned and real-time information at 350,000 GB bus stops (April 2012).
App developers who wish to access NextBuses API — please use the 'contact us' feature at www.traveline.info ... and select the 'NextBuses and txt' option