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 Strathclyde Partnership for Transport, the railway is one of the very few railways in UK remaining in public ownership and 'vertically integrated', where SPT's responsibility covers all aspects of operation and infrastructure.

The railway forms a small circle in the centre-west of Glasgow. All of the passenger railway 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 railway. The route, though nominally 'circular', has many directional transitions creating some further constraints on the operation. There are also significant gradients (up to 6%), 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 freely-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.

The Subway provides close interchange with the suburban and national rail network at Buchanan Street, Partick and St Enoch stations. Bus stances at Govan and Partick stations assist the bus links available from most Subway stations. Park and Ride facilities at Kelvinbridge, Shields Road and Bridge Street stations provide 1,109 car spaces to ease city access by the Subway.

Multi-modal public transport is encouraged through the use of ZoneCard, SPT's season ticket that permits easy use of the Subway, suburban rail and bus services.

In the last year of operation prior to modernisation in 1977-80, 7.34 million passengers were carried. Following modernisation, the yearly figure grew to peaks of 14.70 million in 1994/95 and 14.68 million in 1999/2000 before declining to 13.16 million in 2005/06.

Today, more than 14 million journeys are made on the Subway each year.

The Subway has an excellent safety record. Safety takes highest priority on the railway. Like the large national railways in UK, the Subway must conform to its Safety Case, a document compiling all the railway's safety management details. The railway's Safety Case must be accepted by Her Majesty's Railway Inspectorate (part of the UK Health & Safety Executive) before operation is permitted. The document is subject to regular scrutiny and review, and must be resubmitted every three years.

Many safety features are built into the railway. These include the signalling system, totally renewed in 1996. The stations conform to legislation required for prevention and control of fire. Minimum staffing levels in stations also conform to this demanding legislation. Emergency procedure exercises are regularly undertaken.

Read more about safety and security on the Subway.

A depot at Govan on the extreme west of the route provides maintenance and administration services. Trains access the tunnels for passenger service from the depot by ramped tracks.

System Control at the depot is staffed round the clock, and provides an overview of the whole railway operation. This includes train location and identity, backed up by CCTV images of station platforms.

The sophisticated signalling system is controlled from here. The System Controller has radio communication with all train drivers.

Public address facilities enable announcements to customers in stations and in trains. Traction power supplies are remotely controlled from the System Control centre, and the centre also monitors tunnel water pumps, station alarms and other essential functions.

The railway is covered by an extensive network of CCTV cameras. Each station has viewing facilities for those cameras local to the station. All images can be viewed at the depot control centre, and are routinely recorded there for safety and security.

SPT's Subway workforce maintains most of the railway. This includes the trains that are subject to extensive routines of preventive work and intervention as necessary. Maintenance of the signalling system, the ticketing system, the communications and station services requires SPT staff on hand during all periods of train service. Escalators and water pumps are also included in the range of assets maintained by SPT staff.

Tunnel railway operation requires the track to be maintained at night when no passenger trains are in service. Maintenance ranges from routine replacement of worn rails to visual inspection of the track. The entire track is inspected every night to detect potential defects. Because of the small confined tunnels, little of this track maintenance work can be mechanised. Battery-powered locomotives are employed for maintenance work in the tunnels.