2 rail and 3 rail

You cannot run 3 rail cars on 2 rail track, but you can run 2 rail cars on 3 rail track as long it is flat top 3 rail track.  You cannot run 3 rail locos on 2 rail track, but you can run 2 rail locos on 3 rail track as long as it is flat top 3 rail track and you make electrical changes to the the locomotive.  The exception to that is the MTH 3/2 engines.

Gene Anstine

There is so much available in either - but conversions are not difficult.  If you are starting from scratch, I recommend buying only those compatible with the track that you find most appealing to you.

So if I have Atlas 3 rail track I can run 2 rail non lighted cars on my layout?

Gerry posted:

So if I have Atlas 3 rail track I can run 2 rail non lighted cars on my layout?

Yes, if you modify each and every turnout, by adding styrene shim stock to the tops of each guard rail (the guard rails should be the same height as the running rails). 

   Hot Water touches on it with the guide rails,  but what nobody has specifically addressed, wheel flange and track "code" or height is at the core of the performance issue of 2r on 3r track. The smaller flanges are naturally more prone to derailing and "scale" pieces may still have a code variance. 

  Like other stock, some will perform great, others will make you seriously consider swearing off scale for Lionel pizza cutter wheels.

To use 3r track with 2r locos, the track needs it's outside rails to be isolated to be fed power correctly.  E.g. on Super O the average pieces the rails are connected and it would not work. But isolated S.O. was available and would work.

  There is some aftermarket couplers made with compatibility of even the Lionel lobster claw in mind, and scale to scale odds aren't too bad, but "one size fits all" is not really on the table. May as well plan on getting a coupler height gauge too