3 rail industrial Plymoth switcher conversion

Those locos have small (not tinplate) flanges; not for tubular track.

Why not look for the K-Line Plymouth switcher, it's made for 3-rail track and doesn't have any issue running on any 3-rail track?  Here's one of mine, it has the lobster claw couplers installed.

I have several of the Atlas switchers.  They are very affordable, but then I run them on 2 rail.  The only real change for is putting a scale coupler on it.  I do like the slightly larger Atlas ones quite a bit as well.

For conversion to 3 rail, it wouldn't be impossible, but for the effort Gunrunnerjohn makes a good point about the K-line switchers.  They seems to be selling at a premium these days, but nice little critters.