Originally Posted by: costing 
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There is no relationship between DCC, mfx, 2 and 3 rail. A 3 rail loco or wagon will cause a short on 2 rail systems. Unless you replace all wheels with DC wheels and run ESU locos that can pick up the current either from each side (2 rail / "DC" mode) or all wheels+center (3 rail / "AC" mode). From all other producers they come in either DC or AC. And while DC locos can be relatively easily be converted to AC by adding a slider under a boogie and tying the wheels together, AC locos are much harder to convert to DC because of the solid axle.
Hi Costing,
Thanks for the feedback and comment...
...While I do appreciate what you are saying about shorting with Marklin wagons on two rail ....
...Actually,....There certainly is a relationship between MFX/MM and DCC, and 2 rail and 3 rail:
First there is the obvious.. they are both decoders protocols for train locomotives.
Obvious but important, since both can be run from one controller.
When using Digital 3 rail and 2 rail, both run with DC, not AC, as I understand... in that the decoder converts the power when DC is not supplied.
Which is why the new power adapters for Digitial only feed DC power into the track.
You do still need AC for the analog of course.
Also the both 2 and 3 rail would also share at least one rail as a common ground.
If you should simply shut off one of the rails and used live catenary, a sliderless locomotive would run identically weather it was wired for 2 rail or 3 rail,
as the feed or positive induction rail is the catenary line.
So ... I see quite a few relationships between 2 and 3 and MM and DCC that can be played with ...
...along with the whole history of different decoders are others have mentioned above and their multiprotocol functions.
Calling them Marklin digital "AC" locomotives is a bit of a misnomer,
since its more about the way the DC power is fed to the loco by the rails 2 or 3, slider or wheels.
Oh... and I run my Roco 2 rail passenger cars on the C-track all the time.
The issue for changing the wheels if I want to run them with greater effectiveness on Marklin rail is more about the 0.4mm (or ?) size difference between the rails and the flange size of the wheels that can make my Roco 2 rail cars more likely to derail on Marklin switches than the bigger (not-so-prototypical) Marklin flanged wheels with another spacing. BTW Does anyone know the exact size difference and is the rail distance slightly narrower or wider?
I think Ray mentioned this to me several years ago.
Still thinking of changes Roco wheel sets to Marklin when possible. .. for the above reasons and to increase conductivity for K-track especially.
Feel free to correct my errors with continental frankness.
Peace
Mark Five.