Break-of-gauge

With railways, a break-of-gauge occurs where a line of one gauge meets a line of a different gauge. Trains and rolling stock cannot run through without some form of conversion between gauges, and freight and passengers must otherwise be transhipped. A break-of-gauge adds delays, cost, and inconvenience.

Narrow gauges tend to be associated with smaller loading gauges and sharper curves, which tend to reduce initial capital costs. This offsets the costs of any traffic affected by the break-of-gauge. Historically narrow gauge railways were primarily built on marginal lines, mostly through hilly and mountainous terrain to cut costs and enable any type of rail service at all. Associated disadvantages were not recognized as much as many rail lines were operated independent of potentially connecting lines regardless of gauge as competing companies built and operated them. Only the building of union stations or the nationalization of railroads changed this.

An advantage is that invading armies may be severely hampered (as when Germany invaded the USSR in World War II).

Another advantage might be that if the different gauges have different loading gauges, the break-of-gauge helps prevent the larger wagons straying onto lines with smaller tunnels.

Similarly, if the larger and smaller gauges use different couplers, the break of gauge tends to keep the different couplers separate.

For passenger trains the inconvenience is less, especially at major stations where many passengers change trains or end their journeys anyway. Therefore, some passenger-only railways have been built with gauges otherwise not used in the concerned countries. For example, the high-speed railways (and some rapid transit lines) in Japan and Spain use 1,435 mm (4 ft 8 ¹⁄₂ in) while their respective mainline railroad systems use 1,067 mm (3 ft 6 in) and 1,668 mm (5 ft 5 ²¹⁄₃₂ in).

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