I have looked on the internet for stepper motors and I found these things.
http://www.cs.uiowa.edu/~jones/step/
Big site about stepper motors in general and how to.
http://www.dmicrotek.com/10mmmotors.htm
This would actauly fit my small N-scale diesel loc. There is a 3-16Volt version of it, so you could run it directly from the DCC track power.
I have no idea about how much torque 1.6mNm is (0.23oz.in). But I think torque of stepper is higher then a normal motor of the same size.
There is also a 8mm motor on that site, with a 5V version, but that may use more electronics to downconvert the voltage.
Now, about the electronics, the prototype may be quite large, but may be housed in a car behind it or in a passenger/loc combination.
So, I am thinking of a digital track system, such as DCC, so that it can supply a high voltage to the motor while moving very slow.
first using a diode bridge, a voltage regulator and some caps, suplies the voltage to the CMOS electronics and to the motor. The mass (-) is connected to the mass of the DCC decoder (compared to the motor suply wires, I may need some help from a manufacturer).
Then a 555 timer is set up to run the train at about 50% of high speed, and the voltage reference input is hooked up to the motor supply wires of a DCC decoder with diodes (maybe some caps if the output is not regulated, and a voltage divider 2 resistors).
This will make the pulses from the timer change in frequence depending on the settings of the DCC decoder.
Now, the clock pulse from the 555 timer drives a CMOS decimal counter (the one which will only turn on one wire at the time). It is wired such the the 5th output is hooked up to reset, so it will count from 1to4. The inc/dec input of this counter is hooked up to the front/back light output of the DCC decoder, which will make the counter run forward or backward.
Now, depending on the strength of the output drivers of the counter you may run the motor directly on these four outputs or through some transistors.
So, not that much electronics, and quite simple. And I think a surface mounted version could be as big as a N scale decoder.
And the stepper would be really strong on slow speeds.
Still, my first layout isn't finished yet (I got my first point glued down now). And I don't have the digital components yet. So it might take a few months before I'll experiment with this.
I hope there will be some discussions here what the best design would be, and maybe someone would build such a thing, just to try it out.