mixed gauge
A description of a railway track with (usually) three rails, which enables trains of two different track gauges to run over it.
mixed gauge: three rails for two different train sizes
Mixed gauge is a railway configuration that uses three rails laid in the same track bed, allowing both standard gauge (1435 mm) and narrow gauge (typically 1000 mm or 762 mm) trains to operate on identical routes. The outer two rails belong to one gauge, while a third rail sits between them to serve the other. This engineering solution emerged where railway networks had to accommodate incompatible gauges without complete reconstruction.
The most common arrangement places standard gauge rails on the outside, with a narrow gauge rail positioned on the inside of one of the standard rails. Both train types share a common sleeper and ballast foundation. Load distribution differs between the two gauge systems, which complicates track design and maintenance. The inner rail must be secured firmly to prevent lateral movement under the lateral forces applied by narrow gauge wheels, while standard gauge wheels pass freely on either side of this central rail.
Where mixed gauge appeared and why
Mixed gauge was historically significant in colonial and early post-colonial railways, particularly in India and parts of Africa. When narrow gauge branch lines were built to connect with existing standard gauge trunk routes, mixed gauge sections often formed the junction areas rather than requiring expensive gauge conversion facilities. Australia and New Zealand also employed mixed gauge on certain sections. The arrangement reduced capital costs by eliminating separate parallel tracks or the need for wholesale relaying.
Maintenance of mixed gauge track presents distinct challenges. Wear patterns differ between the two gauge systems, affecting rail renewal scheduling. The inner rail experiences concentrated loading and wear from narrow gauge wheel flanges, requiring more frequent replacement than outer rails. Crossing points and junctions on mixed gauge sections demand precise geometric control to prevent derailment. Most mixed gauge installations have now been retired in favor of unified gauge networks, but sections remain in heritage railway operations.