A passing loop (also called a passing siding, crossing loop, crossing place or, colloquially, a hole) is a place on a single line railway or tramway, often located at a station, where trains or trams in opposing directions can pass each other.Trains/trams in the same direction can also overtake, providing that the signalling arrangement allows it. A passing loop is double ended and connected to the main track at both ends, though a dead end siding, which is much less convenient, can be used. A similar arrangement is used on the gauntlet track of cable railways and funiculars, and in passing places on single-track roads.

Ideally, the loop should be longer than all trains needing to cross at that point. If one train is too long for the loop it must wait for the opposing train to enter the loop proceeding, taking a few minutes. Ideally, the shorter train should arrive first and leave second. If both trains are too long for the loop, time-consuming "see-sawing" (or "double saw-by") operations are required for the trains to cross.