A Marx Gondola in a Lionel Consist

A fairly common conversion that you see in shows and for sale on the net are Marx 3/16th litho cars on Flyer S gauge trucks with link couplers.  I have seen maybe a couple of the reverse....Flyer cars on Marx 3/16 trucks.

will have to look for some of those in the future. nice job.

Yeah, I like to kitbash with "junk" items. An example: I had a surplus of cheap Marx plastic tenders with faux trucks. I cut off the fake trucks and spliced two bodies together, made a metal underframe, added real trucks.

Commonplace junk cabooses with broken corners are good for kitbashes, since they are pretty worthless otherwise. I had one Marx caboose with three broken corners, cut off the roof, squared the ends, shortened the body to fit, made a nifty custom short caboose.

Sometimes I put Lionel cars on Marx trucks, or Marx cars on Lionel trucks, if they look good with the rest of the train that way. Sometimes I use Marx tenders with smaller Lionel locos.

I have a Marx 3/16 tin litho boxcar with the original marx truck with metal fork coupler on one end and a Gilbert American Flyer 3/16 pre war O truck with die cast link coupler on the other. Someone probably did the conversion to run The Flyer and Marx cars in the same train or to use a Flyer loco to pull Marx freight cars 

Always fun to play with Marx; I've dabbled a bit in what you have done - and found more than one of their cars, especially the 3/16" type, on DaBay with the Lionel trucks already on them. Often low-priced - the trucks re worth the asking, sometimes.

Maybe I should take a few shots of mine....

Your gon looks good; On30 is another place to use Marx and other small cars - the "classic" Lionel PW SP-style caboose (billions made!) is a real On30 candidate. Some of the tooling is really rather good in a basic way - a great starting point for narrow-gauge - then detail and paint your brains out.

My one operating S-gauge train has some kitbashed items:
The caboose and green flatcar are prewar AF items, originally O-gauge remounted on S-gauge trucks.
The white tank car is Marx, remounted on S-gauge trucks.
The loco and silver tank car are postwar AF S-gauge unmodified items.
Grouped together they all look close enough to S-scale, although I think of them as On42 alongside my O-gauge.

I have converted a few Marx 7" four wheel tin cars to eight wheel with Marx 3/16 scale trucks

Couple of Marx 3/16" scale metal tankers; the one on the left came already converted to Lionel PW trucks; I painted and decaled it; the one on the right is a work in progress. I removed the 3/16" trucks and added relatively modern Lionel trucks.

I added brake system details as well as the new trucks:

In case anyone thinks that I'm fantasizing or that the car is too, too small...this is from Kalmbach's Freight Cars of the '40s and '50s. (I'm searching for these specific decals, or a set that can be wrangled well enough.) The brake staff/wheel at the end may not be added - no room without frame modification.

Converted a Marx LV hopper. It looks good.