A permanent connection with the return circuit (running rail or return wire) is established by earthing bridges and earthing connectors. The rail potential is also reduced as a result.
A direct connection to the return circuit is only permitted with AC traction systems, however. With DC traction systems, an indirect connection must be established using voltage-limiting devices due to the danger posed by stray currents.
Traction system earthing is therefore always necessary in track systems around overhead contact lines and current collectors (i.e. areas where cables can be split or severed).
In Germany, DIN EN 50122-1 is an important standard for traction system earthing concepts. Specifications from this European standard can be found in the policy "Bahn-Richtlinie RIL 997, Untergruppe 02" (German Rail Directive RIL 997, Subgroup 02), with the title "Rückstromführung, Bahnerdung und Potentialausgleich" (Return Circuits, Traction System Earthing and Equipotential Bonding).