Gauge conversion is the change of one railway track gauge to another. This may be required if loads are too heavy for the existing track gauge or if rail cars are of a broader gauge than the existing track gauge. Gauge conversion may become less important as time passes due to the development of variable gauge systems, also called Automatic Track Gauge Changeover Systems.
If tracks are converted to a narrower gauge, the existing sleepers may be used. However, replacement is required if the conversion is to a wider gauge. Some sleepers may be long enough to accommodate the fittings of both existing and alternative gauges. Wooden sleepers are suitable for conversion because they can be drilled for the repositioned rail spikes. Being difficult to drill, concrete sleepers are less suitable for conversion. Concrete sleepers may be cast with alternative gauge fittings in place. Steel sleepers may have alternative gauge fittings cast at production, may be drilled for new fittings or may be welded with new fittings.
Conversion from a narrow to a wider gauge may require enlargement of the structure gauge of the bridges, overpasses and tunnels, embankments and cuts. The minimum curve radius may have a larger radius on broader gauges requiring route deviations to allow the minimum curve radius to be increased. Track centers at stations with multiple tracks may also have to be increased. Conversion from narrow to standard gauge means several changes not because of the gauge itself, but in order to be compatible with the structure gauge of standard gauge track, such as height of overpasses so that trains can be exchanged. The choice of train couplers may be a factor as well.
Where vehicles move to a different gauge, they must either be prepared for bogie exchange or be prepared for wheelset exchange. For example, passenger trains moving between the in France, and the gauge in Spain, now pass through an installation which adjusts their variable gauge axles. This process is known as "gauge change". Goods wagons are still subject to either bogie exchange or wheelset exchange.
Few steam locomotives are constructed to allow for gauge change. Those that were include some in East African Railways, Garratts, and the Victorian Railways R class R766. Construction of steam locomotives in this way is difficult due to the width of the boilers and the fireboxes. The gauge change is only to a wider gauge because of the widths of the boilers and fireboxes on the s locomotives as opposed to those on .
In about 1860, the Bristol and Exeter Railway converted five locomotives to gauge, and later converted them back again. Also in the 19th century, in the United States, some broad gauge locomotives were designed for easy conversion to gauge. In the 20th century, in Victoria station, London, some broad gauge locomotive classes of the Great Western Railway were designed for easy conversion to a narrower gauge. Between 1922 and 1949, five South Australian Railways T class narrow gauge locomotives were converted to Tx-class broad gauge, and later back again. After World War II, a number of captured German 03 class Pacifics locomotives were regauged to the Russian gauge.
Most diesel and electric rolling stock can undergo gauge conversion by replacement of their bogies. Engines with fixed wheelbases are more difficult to convert. In Australia, diesel locomotives are regularly re-gauged between broad, standard, and narrow gauges.
Gauge conversion of wagons and coaches involves the replacement of the wheelsets or the bogies. In May 1892, wagons and coaches were converted when the gauge of the Great Western Railway was abandoned.
During gauge conversion work, some stations and branch lines may become "gauge orphans". Trains can't operate to them until costly gauge conversion work is completed. For example, between Seymour and Albury in Australia, some branch lines (such as Benalla to Oaklands) and some stations, such as Violet Town have become gauge orphans.