neutral section
A dead section of overhead line for supplying power to trains between two live sections, used to separate two power supplies, different phases and supply voltages.
neutral section: the gap between two power supplies
A neutral section is an unpowered or specially isolated segment of overhead catenary line, typically 40 to 100 metres long, that separates two live sections operating at different electrical conditions. It exists where two separate power supplies meet, where phase sequences differ, or where supply voltages change. Trains must coast or coast and brake through this section without drawing power from the overhead line.
The physical construction varies by rail authority and voltage. In AC systems operating at 15 kV or 25 kV, a neutral section may comprise a dead section with no voltage present, or a transitioned section where the voltage gradually reduces to zero and then increases again under the second supply. The transition is necessary because a train's pantograph cannot switch instantaneously between two AC supplies without arcing and damaging both the pantograph and the line equipment. In DC systems at 750 V or 1500 V, neutral sections are typically fully dead, since DC transitions are more abrupt and arcing damage is less severe.
Signalling and train operation around neutral sections require coordination. Automatic train protection systems must alert drivers to neutral sections in advance. A train approaching at speed will begin coasting well before the section begins, relying on momentum and grade to cross it. On level track or uphill sections, trains may need to time their speed carefully; on downhill sections, regenerative braking (on modern stock) or friction braking may be required. Freight trains, especially heavy and slow-moving ones, can struggle to maintain speed through a neutral section and may lose operational time.
Why they exist and where they occur
Neutral sections are necessary wherever two independent supply sources meet on a railway network. This occurs at the boundary between two traction substations, at junctions where two differently phased AC networks merge, or at the transition between AC and DC lines (such as some Alpine routes). Without a neutral section, the two supplies would try to synchronise through the train's pantograph, causing short circuits, equipment failure, and line shutdown.
Modern high-speed lines and well-maintained metro systems minimise neutral sections through careful substation placement and phase synchronisation. Regional and freight lines, especially where multiple operators share infrastructure, often have more neutral sections and longer ones. Driver experience and route knowledge determine whether a neutral section delays a service or is crossed without incident.