Two-rail / three-rail (stud contact)

In two-rail systems (DC, e.g. Piko, Roco, Fleischmann) current flows through the two rails — polarity matters, hence the reversing-loop problem. In three-rail systems (e.g. Märklin C track) a centre stud contact acts as one pole and both rails as common return — reversing loops are electrically harmless. The systems are incompatible; in practice the choice is made with the track system.