Here are some of my findings of different electrical couplings.

Seeing these new electrical couplings from Märklin and comparing them with others, Märklins couplings seem to be oversized.
I do like the 4 pins instead of Roco's latches.
another important part is whethter or not you can attach wires to the coupling in case they've seperated themselves from the coupling itself.


The Fleischmann couplings are also robust but don't come assembled with wires.
Roco couplings wires, if broken or damaged, can not be easely fixed.
the wire is spotsoldered onto the copper latch and it has a complicated saddle it fits into.
the wire itself, very thin, makes a 90° turn through a small opening.
By soldering it, any excess of solder would prevent the wire or the latch to fit back into its saddle.
Also the latches themselves could get out of line and therefore can be pushed backwards and down or upwards and making it impossible to connect them again.
the lineup of the couplings are sometimes not level and it takes a bit of skill to line them up correctly.

Fleischmann couplings are easy to install and have external soldering pads, making it easy to get too.
when I first bought them, and many of them, they would'nt fit under the buffers and revented the coupling of free side movements.
At an early stage, I cut the coupling and superglued the coupling back on again sideways or horizontal instead of vertical (original).

After another close inspection, I've found the coupling had a couple of hole slots for the wires to fit in, on one end and this prevented the coupling of sliding sideways.
after cutting the plastic "overhang" off, they are working fine in its original design.
Coupling them together is not easy but after a few attempts you get the hang of it.
the stiff wires my dealer supplied me with, are not suitable as they break off from the soldering pad.

this picture was supplied by Tom:





This is a Fleischmann 4 connection coupling


I hope Tom doesn't mind copying the picture

regards.,
John

John - some pictures would be helpful here; do you have any?

I have a Roco 3 part Railcar set from the 80's and it uses 3 pin connectors.

regards.,
John