My Raspberry Pi dream is to add a computer controller to my vintage Lionel locomotives.
The basic design would be to write a PWM program that uses GPIO pins to drive a power converter to control the motor, and use a tachometer on the motor as feedback to the PWM program, again via a GPIO pin.
For this to work acceptably, really two things need to happen. First, the Linux kernel needs to be built with the PREEMPT_RT patch set so that it will support hard-realtime operation. That should allow us to do reliable PWM in software.
The second thing is the necessity of generating interrupts from a GPIO input pin to be able to monitor the tach. Polling really wouldn't cut the mustard.
Obviously you could do this with a $5 8 bit microcontroller, but heck at $25, why not do it with Linux?
Then add a $10 micro wifi USB adapter and maybe even a USB video camera, and a speaker for train sound FX, and you're competing with train control systems that would cost much more. Building a system like THAT from a uC would probably be much more difficult than using Linux with all of it's useful bundled software, and the additional difficulty in adding the extra hardware would probably make it less competitive price-wise. And haven't you always wanted to SSH to your locomotive?