Can someone please explain (simply) what the difference is between AC and DC wheelsets for coaches and wagons?

I have some Roco wagons that I use on my British OO layout.
Would I have to change the wheels to use them on my Marklin layout?

AC is short for three-rail. DC is short for two-rail.

AC wheels on two-rail track give a short circuit.

DC wheels on three-rail track may or may not work, as the wheel spacing may be different. Worth to give it a try.
Contact tracks will not detect DC wheels. If cars derail on turnouts then you will have to adjust the wheel spacing or replace the wheelsets with the AC type.

In my experience there are no problems with Roco cars on Märklin C track.

Basically AC and DC wheels is a misnomer. The real difference is insulated and non-insulated wheel sets. Non-insulated wheel sets will short out a two rail track. Insulated wheel sets have no problems running on three rail tracks but will not activate the contact track which requires power across the outside rails to work. Insulated wheel sets will not activate such a sensor.

There are basically 4 major differences;
1) Insulated vs. non-insulated(AC)
2) 14.1mm spacing vs. 13.9mm spacing(AC)
3) Flat vs. rounded(AC) profile
4) Flange depth


Reg. 1); Goes without saying; AC-wheels for better electrical contact and use of contact-tracks (feedback).

Reg. 2); I´ve seen DC spacing up to 14.3mm, and Märklin M-track that will need spacing as small as 13.8mm. I adjust everything to 13,9mm.

Reg. 3); All M-track, and some K-track is "rounded" on top, not flat like DC-track. The correct AC-profile wheels will thus run better with less derailments and problems.
In most cases you can get by with the DC-profile, as long as you get the wheel-spacing right.

Reg. 4); Most Märklin points and crossings relies on the flange to maintain stability and electrical contact when running through heart-pieces. If the flange of the wheel is too fine, the wheel will "fall down" into the heart-piece and/or may loose electrical contact.
The needed AC flange-depth is quite bulky and doesn´t look nice. C-track has lower rails, and doesn´t need quite so much flange as the older M and K tracks. (But still relies on the flange in some points)


PS: Hm... You might have to substitute "heart" with "frog" in my text.

Thanks Henrik and Hess.
If I come across such problems, I'll know what to do.
Cheers!