Unit/loco identities rather than just train formations.
Yes. And the data about coaches (etc.) should have their door positions (to allow you to know where to stand) and locations of facilities, plus, for applicable stock, the types of seats and where they are.
For example, if I board a commuter train (e.g. a typical Class 377) on the xx:xx service from XYZ to ZYX, I'd like to know where the nearest seat with armrests will be, which is the optimum distance for exiting the platform swiftly, and in addition is not too close to the wonderful smells of an inevitably unflushed toilet. I may also want to specify a nearby luggage rack and perhaps a table for four. In reality, this likely would not work, since the services are generally too busy for this to be planned without reservations. But it's a nice idea.
The clever bit would be updating this according to your detected position in the train, real-time running information and the likelihood of each carriage being full (e.g. where entrances onto platforms have been at each station en-route, thereby allowing you to avoid coaches full of standing latecomers).
Real usefulness could be added by allowing "priority seat" passengers at unstaffed stations to find the right door for a seat that it is suitable for them, without them having to walk too far down the platform or in the train.
IIRC, there are websites with seating patterns, but I am pretty sure it has not been integrated with timetables and the location of various station facilities. Station facilities are mapped in some detail on NRE.
For a somewhat less complicated but very effective example of what I mean, look at the "Tube Exits" iPhone application.
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