DC wheelsets have been covered in this forum before but the information is spread across a number of posts. I’ve just modified some carriages so I’ll put what I found into one post and maybe that will help some other beginners.

Marklin has its own unique wheel standard called NEM 340, most of the “normal” manufacturers seem to use NEM 310 as a base. Wheelsets made to NEM 340 are often called AC wheelsets to reflect Marklin’s original 3 rail/stud track and AC power system while the others are called DC wheelsets.

Here’s the standards in German from https://www.morop.org/index.php/en/.
nem340_d.pdf (41kb) downloaded 24 time(s).
nem310_d.pdf (68kb) downloaded 20 time(s).

If you’re a Marklin user that doesn’t sprechen then this online German to English pdf translator can be handy https://www.onlinedoctra...german-to-english_de_en.
Here’s the standards translated.
nem340_d.de.en.pdf (88kb) downloaded 57 time(s).
nem310_d.de.en.pdf (128kb) downloaded 30 time(s).

As can be seen in the following picture showing Roco DC vs Marklin AC, the major differences are that DC wheelsets have a shorter axle, a narrower tread as well as a lower, sharper flange. Obviously, DC wheelsets must be insulated.



This difference means that DC wheelsets sometimes have running problems on Marklin track, particularly going through point work. Here’s a slow motion video of a six carriage test train going through a Marklin 24671 curved point. The first three carriages have standard Roco DC wheelsets and can been seen to catch on the nose of the frog and bounce up - making quite a racket in the process. The second three carriages have been modified and can be seen to pass through without much drama.

Slow Video 10MB.mp4 (10,677kb) downloaded 63 time(s).

Clearly the best fix for this problem is to replace all DC wheelsets with the correct AC wheelsets. However, the high price and poor availability of the correct AC wheelsets has meant that some people have found a workaround by simply reducing the spacing between the wheels, called the back to back measurement, by 0.5mm. For the Roco DC wheelsets I have that means a reduction from 14.3mm to 13.8mm. The wheels are a push fit on the axles and are not that difficult to get to move. However, getting them to end up at exactly 13.8mm apart and guaranteed to still be “square” on the axles is maybe not so easy.

With 20+ axles to do I decided to 3D print two small precision depth cups with the right recesses to take the ends of the axles. Placing a wheelset to be modified in them and giving two VERY LIGHT taps with a hammer can quickly turn out the desired result with all the finished wheels measured at 13.8 +/- 0.03mm. There are many ways the same result could be achieved. Note that this workaround still leaves the issue of the insulated wheels not operating some detection tracks.