I have been following this thread with interest and am very interested in a "Lego train brick".
Or maybe even a more general PF brick. Unfortunately I don't have the time to help develop it right now and it seems several of you have much more experience with this stuff than me so I think you will do better without me!
What functions do you have in mind for this brick? I have some ideas. Maybe I am just stating the obvious and then you can ignore me...
In a layout you can have several small hubs that can take commands from a central point, perhaps with Bluetooth from a computer?
The hubs can:
1. control PF much like the IR-receiver today (operate for example a train motor).
2. control servos or something to operate switches.
3. forward signals from external sensors (pressure, position? ...)
What protocol is used in Mindstorm NXT? Can that be used to communicate with sensors and servos? Then we already have several peripherals to use and we can make some more.
To know the location of a train RFID can be used with tags on the rail and a RFID reader in the train.
If this would be possible to implement in a small brick I think it would be very useful for trains and even more for Mindstorms and Technics.
I said that I don't have the time but this would be so interesting so I don't think I can keep away. I am a programmer so maybe I can help with the PC software?