Lithium Battery???

I have recently tried using rechargeable LI-ion 14500 batteries. they are the same size as AA, slide right in there and the trains scream. the only problem is that they are too fast sometimes. I got a 'smart charger' on ebay for 20$ and a bunch of the 14500 batteries (and some NIMH AA and AAA batteries) for less than $1 each. I also got some AA to C plastic battery adapters to use in the trains that take C batteries. It all seems to be working well. The 14500 battery gives off ~3.7 V and best I can tell the 130 motors all seem to be rated for ~3-4V.
I'd rather recharge batteries then buy and replace the motors.
KM