Hi Stephen
Not sure which Dampfwagen you talk about.
I have the Kittel 3425 Dampfwagen in a brown livery with a metal case, it is a steam unit which I converted into MFX.

Looking very closely, the Kittel has one traction tire which makes that when running on contact track (I use a lot on my Rocrail Layout) and depending on the orientation, the rail return may occur only by one wheel. This may explain stops when there is anything on the track.
If you have the same unit, maybe adding a rail slider on each side would help for better pickup
Cheers
Jean

I just looked at the bottom from a photo and it actually has a one piece slider that connects to the middle pins on one end, but it also has 2 copper "sliders" on the inside edges of each of the 2 sets of wheels. I would post a photo but I don't know how to do that. Could you help with that?

Yes I cleaned it, and as I said it cleared up in 2-3 other sections of the loop, but this one section hasn't improved. If its "dirty", is it the rails that are at issue or the third rail posts down the center that are the problem?

Mine is M Track. I'll take out a few pieces and see what I can find. Is the "pickup shoe", the long one running in the center on the studs that floats between the wheels?

Well I've removed 4 sections of track, one of which had a feeder line attached. I checked them out, and can't see any issues. I replaced the section of track where the feeder strip connects, but also tried it without the feeder attached, and it cut out at the same place. So I reversed the direction of the car on the track so that the slider is at the rear to see if that made a difference and nope, still the same. So I held the whistle on as in approached the spot on the circuit and it sounded great and cutout at the same spot, and started blowing right after it passed the spot. So its not just the chugging sound that goes out, other sounds go out too, but resume after passing that section. There's no feeder at that point. I tried to bend the slider slightly to see if it was a contact issue with the center studs, and maybe I didn't do it very aggressively since I don't want to break it, but that didn't make a difference either. So I'm at my wits end. Its got to be a connection at that section of track. The 2 pieces fit together tightly, but there is probably a 1/16" gap between the inside rails of that section. But I don't know how I can close that gap in the rail, since the plastic of the track bed is butted tightly. The outside rail has very little gap. Any ideas on this Christmas Eve? Thanks and Happy Holidays.

Thanks Jean for your input. Since the problem occurs only at this one spot, on a big loop layout, I have to assume the slider is fine or otherwise I'd have it occurring in other places. So it must be in this one section of track. Given my limited skills, I think the easiest thing is just to put in a new section or two of track. Hopefully that would resolve it. Still can't figure out why the rails have a wider than normal gap in this area. Thoughts on that?

Thanks. I was trying to test it the other day with a multimeter, but not sure which rails/studs to test, with which DMM leads, and whether AC or DC. Could you help me on this? Thanks.

WD40 is generally not recomended. for more information please read https://www.newrailwaymodellers.co.uk/Forums/viewtopic.php?f=8&t=24595

this video is quite informative

Thanks everyone for your input. Hopefully I'll get it figured out by the end of this year!! Best.

Well I used my Digital Multi-Meter set to Continuity/Sound mode, and tested the M Track sections before and after the point where the sound stops (including testing sections before the trouble point connected to sections after the trouble point), and everything is fine. I've got continuity on the outside rails with no breaks, and continuity on the center studs. The rails are in continuity with each other but not with the center studs, so that's clean. And the loco runs fine with no change in speed when it crosses this spot. And as I said before, the sound car runs fine in every spot but this one. I do have an oscilloscope that I might hook up somehow, and see if that tells me anything. I"ll keep you posted.