Couplings for Liliput H0e and H0m coaches, which will give you magnetic coupling with 2 x 2 mm cilinder magnets (to be purchased separately). These have to be pressed in. You can use them without wires for normal magnetic coupling, but when you solder wires to it, you only need to have electric contact to one pair of wheels to power the whole train.
More info and to purchase magnets and wires see:
https://tramfabriek.nl/couplings.html