I've become interested in Marx trains just in the last few months and have only a few pieces so far, for under $20 total. It's interesting to note that the Marx tab and slot couplers can mate with their knuckle coupler and also with the slot in the tilt coupler. So they maintained some compatibility even with completely different couplers.
I think the beauty of Marx trains is the simplicity of design that made them inexpensive and affordable, with lithography that made them interesting. The tin cars are lightweight and roll easily and have excellent play value. I like the way they rattle down the track. I should have had some when I was a kid, so I'm making up for it now 50 years later.
I bought a bare 4-wheel Marx chassis for a buck, and it was easy to motorize with a vertical shaft DC motor and worm driving directly on an axle gear, using welding rod for the drive axle. Parts all from my scrap box. It's battery power with a homemade boxcab shell.