Originally built for the Glasgow District Subway Company, the railway first opened in 1896 as a cable-hauled system. Propulsion was provided by stationary steam engines and the railway was hailed as the first of its type in the world. The Subway is generally recognised as the world's third underground railway, after London and Budapest. In 1923, the Subway passed into the hands of Glasgow Corporation Transport Department, and in the following decade the railway was converted to electric traction, introducing a third live rail for the purpose.
The railway ran with little further change until 1977 when the new operators, Greater Glasgow Passenger Transport Executive, closed it for major modernisation investment. The railway in its present form reopened for operation on 16 April 1980.
Now part of SPT, the railway is one of the few in the UK remaining in public ownership and 'vertically integrated', where SPT's responsibility covers all aspects of operation and infrastructure.
The Subway forms a small circle in the centre-west of Glasgow. All of the passenger route is underground, contained in twin tunnels, allowing clockwise circulation on the outer circle and anticlockwise on the inner. Fifteen stations are distributed along the route length of just over 10km. Eight of the stations are north of the River Clyde which dissects the circular route.
Most of the tunnels are relatively shallow, following the alignment of the city streets. The deeper sections that pass under the rivers are lined with cast iron segments, whilst the majority of the tunnels are formed from concrete and brick.
Tunnels are very small, at only some 3.4m in diameter. This is in scale with the unusually small and unique track gauge of 1220mm (4 feet from rail-to-rail), and represents a major constraint inherited from the early origins of the Subway. The route, though nominally 'circular', has many directional transitions creating some further constraints on the operation. There are also significant gradients, and the tunnels generally rise to stations and fall to low points between.
Trains are normally formed from three cars, the maximum length of train that can be accommodated in each station. Thirty-three 'power cars' from the 1977-80 modernisation form the majority of the rolling stock fleet. These are self-contained cars, each with a driving cab, controls and motorised to be able to be driven independently. Eight additional 'trailer' cars were provided in 1992 and these have no cab or traction motors.
A normal three-car train is coupled from a power car at each end, and either a trailer car or freewheeling power car in the centre. The three-car train has a seating capacity of some 112 places, and space for additional 165 or so standing passengers.
Each car is very small (12m long), in keeping with the tunnels and the curvatures of the route served. Trains in passenger operation are semi-automatic, where the drivers allow Automatic Train Operation (ATO) to fully control speed and stopping in stations. Under ATO control, trains have a maximum speed of 54 km/h, but are automatically limited to lower speeds for the tighter curves and other route limitations.
Prior to the 1977-80 modernisation, the 15 stations were very basic and similar at track level, having a simple island platform serving either direction of travel. Modernisation introduced separate flank platforms to six of the busier stations. At the same time, 28 sets of escalators were provided, even though there are no deep stations in the Subway.
All stations have staffed ticket offices, normally at street level, and access to platforms is through controlled turnstiles operated by the magnetically encoded tickets. A flat fare applies to any journey on the Subway, and exit from the station is by rotating turnstiles. Vending machines are also provided for ticket sales. The three busiest stations are Buchanan Street and St Enoch in the city centre, and Hillhead to the west.