after years of following Märklin mailing lists and online groups and fora, I have come to the conclusion that the majority of threads about trains not working well end up being because:

a) they were not using original Märklin wheel sets
b) they were not using original couplers
c) they were not using Märklin rolling stock.

I have K tracks and only run Märklin rolling stock. I do not have reproducible derailments.

My experience with C track is that Roco generally runs fine. I think that I've only once have had to adjust wheel spacing on an axle on a three axled auto transporter.
With brands like Brawa and Piko and several Danish brands I sometimes have to adjust or change wheels.
I have no idea if my C track experience is of any use on K track though. I also don't know about the brands that are more readily available in North America.

Thanks for that Roco is available and tickles my fancy often.

I’ve been running Roco on Märklin k track. They have run well with both AC and DC wheels. They are relatively new production and seem to work well.

I run many brands besides Marklin but they all work best with the proper AC wheelsets. Roco says their dealers will swap in the AC wheelsets. I have found they will if they have them. Couplers and buffers are another story, not all brands work well when fitted with the Marklin close coupler.

Roger

Sounds like a good plan.

And yes I will make sure to have wheel set for Marklin if I cannot resist buying other brands.

I run all the brands: Brawa, Roco, Fleischmann, Liliput, Piko - rollingstock and locomotives - on my Marklin K track with CS2 without problems. A number of rollingstock items still have DC wheelsets too as I don't have enough AC to change them over.

My only 'issue' I've had is wagons with DC wheelsets have a little 'wobble' over DKW (2275) points due to the Marklin design of the wheels actually driving on the flange tip for the crossover instead of on the rails. They still don't derail though.

The derail issues I've had is always related to axle width, usually too short and often causes momentary short circuits too. Easily fixed by just increasing the axle width. I think it's only happened when I've converted DC wheelsets to AC on locomotives front axles in order to trigger S88 detection (annoyingly often DC manufacturers still use DC wheelsets on the front bogie on locomotives sold as AC, they drive fine etc. but it means the front bogie wheels don't trigger S88 leading to automation overshots)

The problem occurs to me when backing into a fiddle yard or depot tracks and you're pushing the through a turnout that turns right or left. There is quite a large gap (on C track at least) at the frog so a tiny flange is more likely to get astray. Also if pulling a long ,heavy train through a curve cars with small flanges in the front end of the train can have a tendency to be pulled off the inside rail of the curve.