trailable
Of a system of points: arranged so that the train may run through them in a trailing direction, even if it is not set for that route.
Trailable: points that let trains pass through safely in any direction
A trailable point is a set of railway points (switches) designed so that a train can pass through in the trailing direction, moving against the direction of the points' blade, without derailing even if the blades are not actively set for that movement. This is a safety feature that prevents catastrophic derailment if a train approaches points from an unexpected direction or if signalling fails.
The mechanism works through the geometry of the blade and crossing angle. When a train enters a trailable point from the trailing end, the wheel flanges are guided by the fixed blade and the stock rail rather than by the moveable blade itself. The angle and taper of the blade must be precise: typically around 1 in 8 or 1 in 10 for main-line points. This geometry allows the wheel to climb onto the correct rail without snagging on the blade tip, which would otherwise cause derailment at speed.
Trailable points are mandatory in certain high-risk locations: at the exit of sidings onto main lines, on converging lines where mixed traffic flows occur, and on running lines where a train might encounter unguarded points. Non-trailable points, by contrast, will derail a train that passes through them in the trailing direction if the blades are not in the correct position. These are acceptable only in fully signalled, controlled environments such as yards where every movement is authorized and observed.
The engineering cost is higher for trailable points. The blade must be stronger and the geometry more carefully manufactured to ensure the wheel guidance is reliable under impact. The swing of the blade is also mechanically constrained to prevent it from moving under the impact of a trailing train, which adds to the complexity of the operating mechanism.
Maintenance of trailable points is critical because wear on the blade and stock rail affects the wheel guidance angles. Regular measurement of blade wear, rail profile, and crossing angle is necessary. If the geometry degrades, the point loses its trailable property and must be replaced rather than repaired.