Installing an ERR AC Commander for a Marx 666 engine.

Given that's a 3.2A trip PTC, that motor is probably drawing quite a lot of power.  I'd first do some maintenance, clean the brushes and commutator and lube everything and see if that helps. I'd want to measure the current going to the track and not guess at what's happening here.

I strongly suggest you consider picking up a clamp-on meter if you're going to be doing stuff like this, a great one I use is the BSIDE DC Clamp Meter 6000.  It is a very useful tool and not that expensive, I did a quick review of it here: Clamp-On Meter That Measures DC Current

The feature I was impressed with, besides the very decent accuracy, was it's ability to measure DC current using the clamp-on without breaking the wire.  This is a VERY unusual capability in an inexpensive clamp-on.

I thought of that just as I sent it. I tested with a Fluke i410 AC/DC Amp clamp, and a Fluke 189 meter.  I measured 2.5 Amps, clean or dirty.  I pulled out 4 other Marx engines and checked current of each, they all measure 2-2.7 Amps.

I pulled out some Lionel engines, and they  measured .8-1.7 Amps. 

The AC Commander should handle that, but a 1.6A PTC will eventually trip on 2.5 amps, even if it doesn't get to it's trip rating.

Understood, thank you.