keitaro,   have you looked at the mechanism that the disney monorail guy did. it looks to be the cleanest design to make this work. gearing also to get the most torque out of a smaller motor.   http://monorail.suzieandbob.com/n.html   i corresponded with the chap some when the greenmax monorail was first announced. he was interested in it and was willing to talk to anyone who might want custom mechanisms made for them. they aint going to be cheap, but it looks like he does very nice work.   i also saw a picture of a monorail mechanism someone had kitbashed in japan and for the non powered cars they used a set of rail wheels with the wheels flipped so the flanges were on the outside and the wheels ran along the top edge of the monorail girder. seemed like an elegant solution for a very free wheeling ride. would have to make your own shorter axles and then a simple skinny truck to pop them in, but then the trucks could just be mounted then to the inside roof of the car (with the appropriate spacer block to set the car tot he right height).   making mechanisms like these takes some machining skills and equipment though. i finally tabled the project as i realized that while it was doable and probably w/in my skills and such, it would take wayy too much of my modeling time to do and just not be worth it to me personally that much at this point in time.   cheers   jeff