Hi, doug,
Welcome to the forum!

According to the product database the cars have NEM pockets.
So you only need Märklin E701630 (bag of 10) or Märklin 7203 (bag of 50).
Some dealers will sell them individually.

The cars have RP25 wheelsets. You could also buy the normal Märklin wheelsets 320 551.

Hello Doug!
And Welcome!

If you need the opposite, and/or don't want or couldn't exchange the couplers of your Trix wagon, Kadee have the 'conversion' coupler to Märklin.

You could roll your Trix wagon between two others Märklin wagons with one kind of below couplers installed on each.

http://www.kadee.com/conv/hocc_marklin.htm
http://www.kadee.com/htmbord/page18.htm

Kadee produces 4 different sizes, e.g., #17, #18, #19 and #20 (exchange the page number above accordingly to see a picture).

Cheers,

Walter

Hi Doug and welcome!

In my years of living in the US I converted many NMRA /US spec vehicle to the Märklin system, so I had the opportunity to research on the difference and find a reliable solution.

Tom (aka HO) pointed correctly to the fact that NMRA wheel sets are insulated and what this can do to the operation of a 3 rail layout)

The mechanical aspect is actually more serious.

The key difference is that your original RP 25 wheels have a shallower flange depth. They are designed to run on track and switches with a lower rail profile

On normal track (as in "no switches") these flanges are not a problem as long as you have perfectly laid track, no knicks,etc.

On Märklin switches and double slips , where the flanges support the weight of the car as it passes over the counter rail section, the wheel will "dip" a few tens of a mm, making for a "bumpy" ride.

Unfortunately, the problem there is not only "cosmetic" : in many cases the car could derail ...and often the wheel tyre will short by hitting the adjacent center rail stud.

Using the proper replacement axles on your Trix Boxcar will definitively solve the problem.

On other brands, it is not always possible to find the proper replacement axles: either the axles have the wrong shape and/or dimensions or there are interferences with the deeper flanges and the vehicle body.

But you might get lucky: I recently bought 20 pieces of UP 40ft livestock cars from Athearn...and lo and behold , all my theories failed : with their original RP25 wheels, the cars run flawlessly on my (C-track) layout!

Model railroad is fun: it still brings some randomness to our lives!

Cheers

Kadees are wonderful couplers, but I'd have thought that the trip pin would hit the centre pickup studs on pointwork. There wasn't much clearance between it and the rails when I used them on DC a few years ago, and that was after I'd set them all up properly using the height gauge.

Thanks for all the helpful advice guys!