The route crosses and recrosses the Surrey/Hampshire border, serving the towns of Aldershot and Farnham, before reaching its present-day terminus at Alton in East Hampshire.

Steam trains connected to the Mid Hants Watercress Railway (by way of rolling stock supply or special excursion) operate on the line.

It runs for 17 miles 29 chains (27.9 km) from the South West Main Line at Pirbright Junction in Surrey to Alton in Hampshire.

[3] At Pirbright Junction, a flyover allows trains heading from Alton towards London to pass over all four tracks of the South West Main Line.

[12] The LSWR opened the double-track line from Guildford to Ash Junction on 20 August 1849, when the South Eastern Railway (SER) began running Guildford–Reading services on behalf of the RG&RR.

[28][31][e] The railway historian R.A. Williams, notes that the LSWR and SER "worked uneasily together" as the two companies competed for passengers travelling from west Surrey to London.

[33] In 1858, the SER offered a free bus service from Farnham town centre to its station at Ash in the hope of attracting custom from its rival.

[36] The independent Mid-Hants Railway proposed the extension of the line west from Alton, with the intention of providing a direct link between the army camps at Aldershot and the ports of the south coast.

[40] A horse bus service to Farnham began that year, but the town was not directly connected to the passenger railway network and the closest stations were at Tongham, North Camp and Farnborough.

[42] The original intention was for the line to pass through Redan Hill in a cutting, but a tunnel was added to the scheme in 1866 at the request of the War Department.

[9][42] The army also insisted that the bridge over the Basingstoke Canal at Ash should include a 6 ft wide (1.8 m) untracked section to allow military personnel to cross the waterway on foot.

[44][45] The new line not only included new stations at Ash Vale and Aldershot, but also provided a more direct link to London from Farnham than the existing route via Guildford.

The new route provided a direct rail link from Aldershot to Camberley, the closest station to the Staff College for the British Armed Forces, although initially the majority of services on the southern part of the new line ran as shuttles between Frimley and Farnham.

As part of the first phase of these works, a flyover was opened at Pirbright Junction on 30 June 1901, allowing trains from Alton and Aldershot to pass over the main line on a bridge.

[49] The 4.5 mi (7.2 km) Bordon Light Railway (BLR), which branched from the Alton line at Bentley station, opened on 11 December 1905.

During the First World War military traffic on the BLR was heavy and on Sunday evenings in 1916, there were four direct trains to Bordon from London Waterloo via Aldershot.

[48][65][h] The section closed completely to passengers on 4 July 1937,[65] and during the Second World War, wagons were stored on the line between Tongham and Farnham Junction.

[68] Although electrification between Pirbright Junction and Farnham could be justified on commercial grounds, the continuation to Alton was permitted by the Treasury with the aim of reducing operating costs.

[26] Passenger services were withdrawn on the Meon Valley Railway on 5 February 1955,[82] although freight workings continued between Alton and Farringdon until August 1968.

The Beeching report, published in March 1963, proposed that passenger services should be withdrawn between Alton and Winchester,[83][84] and this section of line closed, almost a decade later, on 7 February 1973.

[96] On 25 May 1985, the Watercress Line, which had started to run heritage trains between Ropley and Alresford in April 1977, began operating from Platform 3 at Alton station.

[97] In 1982, the railways in Surrey and Hampshire came under the control of the London and South East sector of BR, which was rebranded to Network SouthEast (NSE) in June 1986.

[98][99] From 1989, the Alton line was part of the South West subsector of NSE and trains were run under the "Solent and Wessex" route brand.

[107][108] In 2017, the franchise was won by South Western Railway, jointly owned by FirstGroup and MTR Corporation;[109] services were transferred to the new operator on 23 August that year.

[110] Infrastructure improvements on the route in the 2010s included the resignalling of the entire line in 2013, with control transferred to the Woking Area Signalling Centre.

[117] In December 2024, the Department for Transport announced that it would transfer passenger operations on the Alton line to public control in May 2025, when the SWR franchise expires.

[123] An articulated rubber-tyred diesel railcar was tested on the line in May 1932, but it limited passenger capacity and inability to operate track circuits meant that it was not used in regular service thereafter.

[134] Following an extensive overhaul in 1977, Class 487 units were tested between Farnham and Alton before re-entering passenger service on the Waterloo & City line.

[141] Although the line to the gas works closed in the mid-1950s,[48] goods trains carrying agricultural produce continued to run twice weekly from Tongham.

[143] The public goods yards began to close in the mid-20th century, with the withdrawal of facilities at Farnham on 4 May 1950,[144] Bentley on 1 June 1964,[27] Alton on 6 January 1969[27][89] and at Aldershot on 6 October 1975.