Hello Philippe
I have done quite a few Kato US locos conversions to Märklin 3-rail system, both for customers and my own collection..

Installing the pick up is **not** trivial.

After many years of trials and errors, the only viable solution I have settled for is:

1° Disabling one of the power bogie (=removing the gears) to find space for the pick up shoe, then adding some custom hardware to fit the slider.
2° Fitting 4 wheels of the remaining powered bogie with traction tires. This means taking the wheels off the axles and cutting a groove on the lathe to install the tires, then re-assemble with accurate spacing on a wheel press

Some locos worked well with their original RP 25 wheels (like my EMD GP-35) , some didn't (like the EMD F-3 A and B) Don't know why. Just a fact.

In that case, I replaced the original RP25 wheels by ROCO NEM wheels (easy to get and cheap, and above all the right diameter) , which then implied cutting all new axles to fit these to the Kato bogie .

Sometimes also then, small adjustments to the bogies' frames details (like brake shoes) may be required.

Not overly complicated, just a somewaht long and meticulous process.

Result is a loco with superior performance to the original (at least twice the pulling power) and smoother than most Märklin locos.

If any 2-rail locos deserve all that fuss to run on a Märklin layout, Katos are the ones!

Pictures available if you are interested.

Cheers

Jacques

PS: I no longer perform these "surgeries" professionally!!

I wonder if it would not be easier to install a slider on the car behind the locomotive and run wires to the decoder in the locomotive.

Some of the Atlas and other models made by Roco may allow for easier conversions

You might also want to check as there were one or two companies that did make American prototype for 3 rail AC.