Use of Rheostat or Potentiometer

I don't know that the pot will be all that useful with locomotives with speed control, not to mention they shouldn't need it.  If you have issues with PS/2 or PS/3 locomotives slowing on grades, I'd look at power distribution first.

Half (11) of my engines are Lionel PW, 70s, and 80s, the other half are MTH PS1,2 and 3s. I don't have DCS and I run every thing in conventional mode. My power source is a Lionel PW ZW and KW.  

Well, even without DCS, the PS2 or PS3 stuff should have working cruise control.  If they're having trouble with grades, I suspect you may have power drops in the feeds to the track.

One more effective way to deal with grades is use a separate transformer for the grade and set that one at a higher voltage to give more power for the grade.  That's what Bill Henning did for his Christmas Tree that was featured here and in the OGR magazine.

If you want to use rheostats or fixed resistors, you'll need very high wattage capability to withstand the to-be-expected derailment and short.

Building your own, consider Variacs.  I use a relatively small one ahead of a 24 volt stepdown transformer.  Circuit breakers and fuses in and out.

I've had good results with the old Lionel prewar #95 rheostats/resistors to reduce power on downhill track sections and to slow engines to a stop gradually with my #132 automatic stop station sections without losing the E-unit setting.  There's also a #81 rheostat but I have no personal experience with that model; looks like it should work too.   They seem to have enough wattage to handle my postwar conventional trains.  They're usually about $10 at shows and on the bay.

Fj