I'm working on building a custom control interface for a model train set my dad is building. He's gotten pretty ambitious with the track layout, and I know I'm going to need more than 20 i/o pins on the arduino for all the electrical elements I'm going to have to control.
Essentially, we're going to be using multiple arduinos (or maybe just one with a centipede shield) communicating with a touchscreen UI running on some sort of embedded device to control all of the track elements (turnouts, forward/reverse, engine speed, turntables, sound effects), lighting effects (buildings and such), and some safety systems (the layout includes a couple turnaround loops and it's conceivable we'd have two trains on the same loop at the same time - the arduino could shut down an entire loop automatically if it detects a problem).
I originally submitted this for the Nokia n900 push project. Sadly, it was rejected. We're moving forward with the build, though. I'm probably going to re-purpose a TI Beagleboard for the user interface, since it's silent and the serial USB connection with the arduino should be trivial to set up. Either that or a Nokia N810 or a Pandora handheld -- haven't decided which one I think I can spare yet. 
Some of the technical considerations for this project can be reviewed at rootaction.net/~boltonc/train.pdf Ignore the prototype track; we're skipping that step and working directly with the final track layout.
We're also looking into what kind of stepper motor we would need to build our own custom turn-table, and whether it would be more cost-effective than purchasing one at retail.
The project is long-term, but is already under way. We finished building the table top this afternoon. It's sectional in 6 pieces since the overall layout is 10x12 feet. We'll be getting together in another week or two to finish the structural build (assemble legs and casters on each section and bolt them together), after which we'll be working independently; he'll lay the track while I start prototyping circuits. It'll take us a month or two before we're ready to put it all together. I'm not sure if that meets your requirement for 'something plan to build immediately' or not, but I do know the centipede shield will save me some coding and cash.
Even if you don't decide to throw a freebie my way, I'm interested in purchasing one when you're ready to start selling them. Do you know yet when that might be and/or where you'll make them available?