The Southern Railway in the United Kingdom was one of the "Big Four" railway companies set up after the 1923 Grouping. This list sets out the constituents of the Company.

Constituent companies

The London and South Western Railway (LSWR) route mileage 1,020 miles (1,642 km)
The London, Brighton and South Coast Railway (LBSCR) 457+1⁄4 miles (736 km)
The combined systems of the South Eastern Railway and the London, Chatham and Dover Railway, under the South Eastern & Chatham Railways' Managing Committee (SECR) 637+3⁄4 miles (1026 km). These concerns had formed a working union to operate their services under a Managing Committee on 1 January 1899.^[1]^[2]

Subsidiary companies

Originally leased to or worked by LSWR
- Bridgewater Railway 7+1⁄4 miles (12 km)
- Lee-on-the-Solent Railway 3 miles (5 km)
- North Cornwall Railway 52+1⁄2 miles (84 km)
- Plymouth and Dartmoor Railway (portion used by LSWR) 2+1⁄4 miles (4 km)
- Plymouth, Devonport and South Western Junction Railway (except Bere Alston and Callington section as above) 19+1⁄2 miles (31 km)
- Sidmouth Railway 8+1⁄4 miles (13 km)
Originally leased to or worked by LBSCR
- Brighton and Dyke Railway 4+3⁄4 miles (8 km)
- Hayling Railway 5 miles (8 km)
Originally leased to or worked by SER or LCDR
- Cranbrook and Paddock Wood Railway 11 miles 24 chains (18.19 km)
- Crowhurst, Sidley and Bexhill Railway 4 miles 40 chains (7.24 km)
- London and Greenwich Railway 3+3⁄4 miles (6 km)
- Mid Kent Railway (Bromley to St Mary Cray) 2+1⁄2 miles (4 km)

Victoria Station and Pimlico Railway (in which the Great Western Railway and the London and North Western Railway were also parties)
Lynton and Barnstaple Railway: not covered by the Railways Act 1921, but absorbed by the LSWR. Narrow gauge (1 ft 11+1⁄2 in (597 mm)) 19+1⁄4 miles (31 km)
Several light railways, including the Basingstoke and Alton Light Railway, though other candidate lines remained independent, such as the Kent and East Sussex Railway

The information in this article taken from the Railway Magazine for February 1923, except as follows:

^ Dendy Marshall, C.F.; Kidner, R.W. (1963) [1937]. History of the Southern Railway (2nd ed.). Shepperton: Ian Allan. pp. 314, 345, 355. ISBN 0-7110-0059-X.
^ Nock, O.S. (1971) [1961]. The South Eastern and Chatham Railway. Shepperton: Ian Allan. p. 125. ISBN 0-7110-0268-1.

v t e The "Big Four" pre-nationalisation British railway companies
Great Western London & North Eastern London, Midland & Scottish Southern
GWR constituents	Alexandra Docks Barry Cambrian Cardiff Great Western Rhymney Taff Vale Full list
LNER constituents	Great Central Great Eastern Great Northern Great North of Scotland Hull & Barnsley North British North Eastern Full list
LMS constituents	Caledonian Furness Glasgow & South Western Highland Lancashire & Yorkshire London & North Western Midland North Staffordshire Full list
SR constituents	London & South Western London, Brighton & South Coast London, Chatham and Dover Railway South Eastern and Chatham Railway South Eastern Railway Full list
See also History of rail transport in Great Britain 1923–1947 List of companies involved in the grouping