triangle

A triangle is a junction of three railway lines arranged so that a locomotive or train can enter from any one of the three directions and exit from either of the other two. The three legs of the triangle are connected by curves, typically two gentle arcs and one sharper curve, forming a closed loop. This arrangement lets you reverse the direction of a locomotive without needing a turntable or a long siding, making it invaluable in yards, depots, and branch lines where space is tight or turnaround capacity is needed.

The geometry varies depending on site constraints. Each leg may be several hundred metres long, and the curves are banked and radiused to permit safe passage at operating speed. One leg often feeds the main line while the other two serve depot facilities, engine sheds, or branch routes. The interior angles are never exactly 60 degrees: the actual geometry is driven by topography, existing track alignments, and safe curvature radii for the rolling stock in use.

Practical operation and limitations

In operation, a locomotive can enter the triangle via any leg and exit via either other leg. This flexibility means a train arriving on the main line can be turned around and sent back without running to a dead-end siding. However, triangles demand careful track maintenance because the curves carry heavy concentrated loading; the sharper curves especially experience high wear on rails and wheel flanges. Points (switches) at each junction must be properly aligned and kept in good condition, since a facing point movement through the triangle is common.

Triangles are most common on secondary lines and in rural railway infrastructure where passenger or freight services reverse direction at intermediate points. They are less common on modern main lines, where separate turntables or loop lines are preferred. The term itself comes from the obvious three-sided track geometry; it is a purely descriptive name with no reference to any inventor or standard specification.

When a triangle is no longer used, the central land is often reclaimed. The layout can also be partially dismantled if only two legs remain active, though this is rare because the three-way flexibility is the whole point of the installation.