shunt

A shunt is a movable section of rail that physically redirects a train from one track to another by pivoting or sliding to align with the destination rail. The shunt itself is typically a pair of tapering steel rails, called points in British usage, that can be switched between two positions. When a train approaches, the shunt directs the wheels onto either the main line or a branch line, siding, or yard track. The mechanism is operated either manually by a lever, electrically by a motor driven from a control room, or hydraulically on modern installations.

Shunts come in several configurations depending on the speed and complexity of the junction. A simple facing shunt allows a train to diverge left or right; a crossover shunt, positioned between parallel tracks, allows a train to switch from one parallel track to its neighbour. Stub shunts are used where space is limited. Double slips, which are more complex and expensive, allow trains to cross and diverge simultaneously. The angle of the shunt's taper, called the crossing angle, typically ranges from 1 in 8 to 1 in 15 depending on the track class and permitted speeds; sharper angles are used on branch lines and sidings where trains move slowly.

The mechanical loads on a shunt are substantial. The guard rails and closure rails must guide and support wheel flanges safely as they traverse the crossing, and the shunt rails themselves wear faster than plain track because they experience lateral stress. Wear plates are often installed where wheels contact the crossing point. Shunt failure modes include sticking in an intermediate position due to ice, mud, or debris in the mechanism; lateral wear that allows wheels to climb the rail; and lock rod breakage on hand-operated shunts.

Modern railway yards and junctions use remote electrical operation of shunts to allow one operator to control multiple junctions from a signal box or control centre. Safety interlocking systems prevent a shunt from moving while a train is crossing it and prevent conflicting train routes simultaneously. The position of each shunt is monitored by position detectors and displayed in the control room.

The term shunt derives from the verb meaning to push or move to the side, and it has been used in British railway vocabulary for over 150 years. American railways use the term switch or turnout for the same equipment. The broader operation of moving railway stock between tracks and sidings is called shunting or switching, and workers who perform this work are shunters or switchmen.