just out of curiosity, any size, wieght or cost restrictions?
is that 3 to 14 volt dc, used to control speed of motor?
it might be useful to use a PWM instead of changing the voltage. (tutorials on it somewhere, read through the dc motor tutorial, its linked in there I think) though that requires some extra work. and I don't know if it would work on a train track...
theoreticaly its just a giant wire. with assciated effects of amplifing annoying quantum physics stuff and other stuff we like to ignore...
(I may have gone off on a random tangent, which is normal, don't worry!)
most stuff I have found in a quick google search is high torque or high speed, but not both.
do you have figures for torque and current draw?
high torque low current are not things I tend to look at. my style tends to be more attaching it to a giant Pb battery, and having another power supply for the controler.
any reason for not wanting a gearbox?
another option is ringing up one of the suppliers like active robots and asking them if they could advise you on a suitable motor for your requirements?
