I bought a Peco "Jeanette" kit back in 1977, and the Arnold chassis was still available then, so I of course acquired one to go with the kit. To be honest, I found the mechanism unreliable even though it was new. The motor would frequently lift itself loose from the chassis, thus disengaging the gears.

Bealman

I purchased  two 'Jeanette's' (which is how I got into 'N' scale BTW; long story).  The previous owner / builder had evidently a similar experience with the motor lifting.  His solution was to solder a short length of spring above the motor in such a way that is held the motor in mesh when the kit body was in place.  It worked very well and I had no  problems with the motor. 

The valve gear was another story altogether...