Several manufacturers offer both 2 and 3 rail versions of rolling stock. Atlas, Weaver and the old K-Line did. Can 2 rail rolling stock be run on 3 rail track without changing the trucks?

2-rail wheels have smaller flanges to run on more prototypical height track. They are no where near actual protototypical size, but much smaller than 3 rail flanges. Also the wheels have flatter and narrower threads - still angled but flatter.

so to answer your question: It depends.

2 rail wheels will not work on old tubular track with round tops. They also may have problems with loose tolerances on switches.

On the other hand, they will run on properly gauged 3 rail track with flat top rails. I ran 2 rail equipment on Gargraves track in the 70s. It had a much flatter top then. Atlas track probably will work in most cases. You may have to extend or tighten guard rails on switches. But if the track is the right width (gauge), with a good profile including flat top, the cars should run.

Going the other way is much much harder. It is pretty much impossible to run 3 rail wheel sets on 2 rail track without major changes. First 2 rail track is much lower profile, so the pizza cutter flanges will bounce on the ties. but that is minor compared to the shorts. 2 Rail wheelset are insulated because the 2 rails carry the opposite sides of the circuit. 3 Rail wheel sets are not insulated - hence they immediately cause a short when put on 2 rail track.

Not to mention the Klunker Couplers on 3-rail rolling stock won't work well on 2-rwork on 2-rail and the 2-rail couplers may not work on 3-rail.

The biggest differences from an operational standpoint are the couplers and wheels. These of course can be changed. Some manufacturers such as Atlas make the same car available in either 2 or 3 rail version and make it very easy to change trucks or couplers to suit. Typically 2 rail cars will be more to scale proportions but there are always exceptions. You can convert anything to run on any track with a little work though. For the Atlas 3R cars, it is as easy as changing the trucks and couplers and no custom work is reguired. On other products such as MTH or Lionel cars, you may need to go to more trouble to swap them out and some custom work may be required. Keep in mind that many of these cars aren't true scale representations though but a few are. If running on 2 rail track, it's easiest to just buy cars intended for 2 rail operation. If you have some old 3 rail cars that you just want to see run, you can always do the conversion.