Fouling point

The fouling point marks the spot at a turnout or crossing beyond which a vehicle standing on one track would foul the clearance of the other (diverging) track. On the prototype it is a low marker set between the converging rails; on a model it follows from the geometry. It matters for planning because the usable length of a stub or storage track only counts beyond the fouling point — a vehicle stopped short of it blocks the turnout and makes the neighbouring track unusable.