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.