I have a newer br 218, which is essentially the same in weight, and wheelbase. It occasionally stops when driving at lower speeds on a level C track curve of radius 3 or 4. No derailmaents, but it could be your coupling to the wagons of distance between the buffers and the copler hooks. Sometimes they just hang up too.

Hi
Regarding using M track, if it is in good shape (no bent rails / ballast and good rail joiners) and well adjusted counter-rails then there is ZERO problem with Märklin cars and waggon EXCEPT:
- very long cars may collide with swich lanterns (the large ones between 1956 and 1977)
- the R1 bridges may cause problems with long cars.

One critical thing is the wheel spacing Early wheel spacing (betrween flanges) was 13.9mm and is now 14.0 to 14.1 mm.
The counter rails on old M-track points are made of mild steel being inserted into the metallic ballast and anchoring tongues being twisted underneath the ballast.
On switches giving problems, you may loosen those counter-rails and put them back in place (usually closer to the rail) so that the "other" wheel is not bumping on the frog.
Newer M-track points have their counter-rail molded in plastic with the frog and ties. No adjustements there.

Slider length: Märklin certifies all its track pieces using a 50 mm long slider. There MUST NOT be any current pick-up interruption PROVIDED the slider is flat and new and without fixing recess and hole.

Solenoid speed: on C Track an impulse of 100 ms is enough to drive safely a solenoid. With M track points and signals, 200 mS usually will work but sometimes may require a 300 mS impulse.

So, it is sure a 39904 loco will pose NO problem on a good M track.

Cheers
Jean

Dear Jean,
I thank you sincerely for all the valuable detailed information you have shared and for the encouragement about the purchase of the loco 39904.
I find it strange that it is not readily available in the train shops in Spain, except Iberplace.

I have many M tracks I have been using since many years, and now started adding the new C tracks to my layout, by the use of the transition tracks.
I have recently had my analogue locos converted to digital and am using the ESU control panel.

I am planning to buy new locos and that is the reason of my question about compatibility of new locos on M Tracks.

Thanks again

Officially that particular loco is only available in the official Marklin Stores, of which there are about 13 or so in Europe. That is why it is not generally available in all stores. That is why it is called a "store locomotive".