I am selling my 2 rail cz cars and someone asked if they can run on 3 rail track. ?
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The problem arises when the two rail trucks move through the switches. The gap near the switch "frog" is too large for the wheels...if the wheels are "scale" wheels. The two rail (scale) wheels will derail.
If you want to run the CZ cars on Atlas three rail, just change out the wheel sets. Most scale truck side frames will accommodate three rail wheel sets; I've done it many times.
To RockyRoad: how is the lighting affected by changing out the wheelsets?
This is a "qualified" answer. Running 2-rail cars on 3-rail Atlas, Ross, Gargraves or MTH track is fine, but you're restricted as to turnouts. Curve-replacement turnouts generally will have problems because the curve arc extends through the frog. Numbered turnouts (#4, #5) seem to work fine with 2-rail equipment because the track is straight after the points and through the frog. The wheels will dip in the frog, but in my experience, they don't derail, even when backing. I run scale wheel diesels and freight cars all the time at the club. I have a couple of cars that are uncooperative (may have slightly tweaked frames that I can't detect). Rex's 2-rail GGD passenger cars ran fine on the layout as well.
You won't get passenger car lighting unless you use batteries or switch out the trucks for 3-rail (which moots this discussion). One other caveat is that code 172 tread wheels work better because the gauging on Gargraves track can get a bit sloppy over time.
You will have to be conscientious about curve radius, which is half the "O" curve number because you're dealing with body-mounted couplers. You also have to be careful not to couple short cars directly to long ones or the short ones can get derailed from lack of coupler swing.
Won't the CZ cars with lighting short out on three rail track with two rail wheelsets?
Since all the wheels are on outside tracks, there is no power. No shorts, just no lights.
Thanks, GRJ. I tried 3 railing an Atlas 2 rail CZ car with mixed results. I ended up using battery power for lighting. Connecting the wiring to the truck side frames was my undoing. I ended up paying the price on E-bay for the cars I needed.
Well, yes, the battery would have a direct short in that case. 
GRJ: AAA batteries?
If you connect any battery to the 2-rail trucks, and put the car on three rail track, you'll short the power source. Of course, that does assume that the two outside rail are connected, they usually are.
GRJ: I think we are on different wavelengths. I converted to 3 rail trucks (from 2 rail), was unable to connect the wiring, so decided to use the option of 2 AAA batteries for lighting. Works fine.
Well, for 3-rail trucks, you'd need pickup rollers for the center track, did you have those?
