Yes, the items are offered in both brands in most cases, or a d.c. wheel set for the cars. I buy Roco cars and don't change the wheels. They run fine on C track. There is the occasional turnout derailment, but if it is persistent, an easy fix is to affix a this strip of .025"x.055" styrene to the inner/outer (depemding on left/right) frog of the turnout curve with plain white paper glue so it can be removed if desired. You can use .030x.060" and file it a bit thinner. That was the only size I found.

From a quality perspective the locos were like Marklin's

Hi
Trix and Märklin are designed and manufactured at the same place using the same components so the quality is identical

In both cases the wheel spacing and profile are the same but Trix wheels are insulated.
On many recent Trix / Märklin the decoders are the same.
On C track, switches are different (especially near the frog and frog connection)
On detection, 2 rails use 60882 detectors - measuring current from 10 mA - while 3 rails use 60881 with non-insulated axles.
But of course you know all this sorry
Jean

Hi

This is a difficult question for us in Europe as most dealers will exchange the set of wheels for free between Marklin and Trix;
This also means that Märklin has rationalized its wheels (same type in insulated or standard execution for 3 or may 4 diameter.
When hesitating, find a Märklin reference (e.g. 41310) go to the Marklin.de website and select Ersatzteilliste. Its gives you the axle reference for 2 rails or 3 rails for this wagon.
Sadly they don't give you the wheel diameter vs reference and wheels are special (Scheibenbremse, Wheels with diskbrakes)
Doing this a few times on the cars you have and you will notice only a reduced number of wheel references that you could order to yourself if there are no dealers around.

Cheers
Jean

ESU wheelsets are for ESU cars.
Märklin wheelsets come in different sizes. When it comes to coaches, there are many different wheelsets used by Märklin. It's a bit simpler for freight cars.
In my experience Trix H0 cars run on Märklin C track without any problems. I don't use contact tracks and simply leave my Trix H0 cars untouched.
Trix Express is a different story: those wheels will not run on C track.

Some Märklin wheelsets have an RRP of €5. That makes €20 for a coach with two bogies. Keep that in mind before buying Trix Express rolling stock.

Alle trois et' magnifique Jean!

Hi Kimball

On these old models the zamak cast makes only room for the original PCB on both locos and the very large motor.
I am sure I could make additional space by milling the frame but I have no milling machine and I hate to make non-reversible changes on my models.
So as you say "That's that" Sorry.
Cheers
Jean