In railroad structures, and rail terminology, a wye (like the 'Y' glyph) or triangular junction (often shortened to just triangle) is a triangular joining arrangement of three rail lines with a railroad switch (set of points) at each corner connecting to the incoming lines. A turning wye is a specific case.
Where two rail lines join, or where a spur diverges from a railroad's mainline, wyes can be used at a mainline rail junction to allow incoming trains to travel in either direction.
Wyes can also be used for turning railway equipment, and generally cover less area than a balloon loop doing the same job, but at the cost of two additional sets of points to construct and then maintain. These turnings are accomplished by performing the railway equivalent of a three-point turn through successive junctions of the wye. The direction of travel and the relative orientation of a locomotive or railway vehicle thus can be reversed. Where a wye is built specifically for equipment reversing purposes, one or more of the tracks making up the junction will typically be a stub siding.
Tram or streetcar tracks also make use of triangular junctions and sometimes have a short triangle or wye stubs to turn the car at the end of the line.
Considerations
At junctions
The use of triangular junctions allows flexibility in routing trains from any line to either of the two other paths, without the need to reverse the train. For this reason they are common across most rail networks. A slower train may be signaled to temporarily enter a wye (as a refuge siding in lieu of a passing loop) for a meet with an oncoming train, or to allow a faster one to overtake, and then reverse out to continue in the original direction.
Where one or more of the lines forming the junction are multi-track, the presence of a triangular junction does introduce a number of potential conflicting moves. For this reason, where traffic is heavy the triangle may incorporate flying junctions on some of the legs.
For turning equipment
From time to time it is necessary to turn both individual pieces of railroad equipment or whole trains. This may be because the piece of equipment is not directionally symmetrical, for example, most steam locomotives and some diesel locomotives, or where the consist has a dedicated tail end car such as an observation car. Even where equipment is symmetrical, periodic turning may still be necessary in order to equalize wear (e.g., on the London Underground's Circle Line).
Several different techniques can be used to achieve such turning. Turntables require the least space, but can generally only deal with a single piece of equipment at a time. Balloon or turning loops can turn trains of any length — up to the total length of the loop — in a single operation, but require far more space than wyes. Rail wyes can be constructed on sites where a loop would not be possible, and can turn trains up to the length of the stub tracks at the end of the wye.
Railroad systems in North America and Australia have tended to have more wyes than railroads elsewhere. North American locomotives and cars (such as observation cars) are more likely to be directional than those found on other continents. In Canada and the United States, the railroad often was built before other structures, and railway builders had much more freedom to lay down tracks where they wished. Similarly, when not constrained by space limitations many early Australian railways made use of wyes (particularly in rural locations) for their lower installation and maintenance costs; however, their necessity and use diminished from the 1960s onwards with the major trend in most states toward bidirectional locomotives and railcars.
In Europe, although some use was made of bi-directional tank locomotives and push–pull trains, most steam locomotives were uni-directional. Because of land usage considerations, turntables were normally used to turn such locomotives, and most terminal stations and locomotive depots were so equipped. Over time, most diesel and electric locomotives ordered in Europe have been designed to be fully bi-directional and normally with two driving cabs. Thus most rail wyes, where they existed, and turntables have been taken out of use.
Streetcar or tram systems
Similar considerations as for mainline rail systems apply to the use of triangular junctions and reversing wyes on streetcar and tram systems. Many, although by no means all, streetcar and tram systems use single ended vehicles that have doors on one side only, and that must be turned at each end of the route.