The London and North Eastern Railway (LNER) was formed out of a number of constituent railway companies at the grouping in 1923.

Main companies

The main companies, showing their route mileage, were:

• Great Eastern Railway (GER) 1,191+1⁄4 miles (1,906 km)
• Great Central Railway (GCR) 852+1⁄2 miles (1,364 km)
• Great Northern Railway (GNR) 1,051+1⁄4 miles (1,680 km)
• Great North of Scotland Railway (GNSR) 334+1⁄2 miles (535 km)
• Hull and Barnsley Railway 106+1⁄2 miles (170 km) (H&BR) (which had amalgamated with the NER on 1 April 1922)
• North British Railway 1,378 miles (2,218 km) (NBR)
• North Eastern Railway 1,757+3⁄4 miles (2,812 km) (NER)

Subsidiary companies

Independently operated lines

• Colne Valley and Halstead Railway 19 miles (31 km)
• East and West Yorkshire Union Railway 9+1⁄2 miles (15 km)
• Mid-Suffolk Light Railway 19+1⁄2 miles (31 km)

Leased or worked railways

Many of these "railways" existed only in name; there were included on the list at the time of the Railways Act in order to legally qualify each line's position

• Originally leased to or worked by the NER
  • Brackenhill Light Railway (West Yorkshire)
  • Fawcett Depot line (County Durham) 5+1⁄2 miles (9 km)
  • Great North of England, Clarence and Hartlepool Junction line 6+1⁄2 miles (10 km)
• Originally leased to or worked by the GCR
  • Humber Commercial Railway and Dock
  • Mansfield Railway 10 miles (16 km)
  • North Lindsey Light Railway 12 miles (19 km)
  • Seaforth and Sefton Junction Railway
  • Sheffield District Railway 4+1⁄2 miles (7 km)
• Originally leased to or worked by the GER
  • London and Blackwall Railway 6 miles (9.7 km)
• Originally leased to or worked by the GNR
  • East Lincolnshire Railway 47+1⁄2 miles (76 km)
  • Horncastle Railway 7+1⁄2 miles (12 km)
  • Nottingham and Grantham Railway and Canal 23 miles (37 km)
  • Nottingham Suburban line 4 miles (6.4 km)
  • Stamford and Essendine Railway 12+1⁄2 miles (20 km)
• Originally leased to or worked by the NBR
  • Edinburgh and Bathgate Railway 10+1⁄4 miles (16 km)
  • Forth and Clyde Junction Railway 30+1⁄2 miles (49 km)
  • Gifford & Garvald Railway 9+1⁄4 miles (15 km)
  • Glasgow and Milngavie Junction Railway 3+1⁄4 miles (5 km)
  • Kilsyth and Bonnybridge railway 8+1⁄2 miles (14 km) (worked jointly with CalR)
  • Lauder Light Railway 10+1⁄4 miles (16 km)
  • Newburgh and North Fife Railway 13+1⁄4 miles (21 km)
• Originally leased to or worked by H&BR
  • South Yorkshire Junction Railway 11+1⁄2 miles (18 km)
• Originally leased to or worked by several constituent companies
  • Nottingham Joint Station Committee
  • West Riding and Grimsby Railway 32+1⁄2 miles (52 km)

Independently operated joint companies

• East London Railway: jointly leased by the LNER, Southern Railway, Metropolitan Railway (MetR) and District Railway. Traffic operated by MetR (passenger); LNER (goods)
• Cheshire Lines Committee (CLC): Operated jointly by LNER/London, Midland and Scottish Railway (LMS). LNER supplies locomotive power; CLC own rolling stock
• Manchester, South Junction and Altrincham Railway: trains worked by both LNER/LMSR

Joint railways

Now totally LNER

• Great Northern and Great Eastern Joint Railway 123 miles (198 km)
• Hull and Barnsley and Great Central Joint Railway 25+3⁄4 miles (41.4 km)

Joint with LMS

• Axholme Joint Railway 27+3⁄4 miles (44.7 km)
• Cheshire Lines Committee (2⁄3 share) 142 miles (229 km)
• Caledonian and Dunbartonshire Junction Railway (including Loch Lomond steamers) 7 miles (11 km)
• Dundee and Arbroath Railway (including Carmyllie Light Railway) 23 miles (37 km)
• Great Central and Midland Joint Railway 40+1⁄4 miles (64.8 km)
• Great Central & North Staffordshire Joint Railway 11 miles (18 km)
• Great Northern and London and North Western Joint Railway 45 miles (72 km)
• Halifax and Ovenden Junction Railway 2+1⁄2 miles (4.0 km)
• Methley Railway (2⁄3 share) 6 miles (9.7 km)
• Midland and Great Northern Joint Railway 183+1⁄4 miles (294.9 km)
• Norfolk and Suffolk Joint Railway (3⁄4 share) 22+1⁄4 miles (35.8 km)
• Oldham, Ashton and Guide Bridge Railway 6+1⁄4 miles (10.1 km)
• Otley & Ilkley Railway 6+1⁄4 miles (10.1 km)
• Perth General Station (2⁄3 share)
• Prince's Dock, Glasgow (1⁄3 share) 1+1⁄4 miles (2.0 km)
• South Yorkshire Joint Railway (3⁄5 share) 20+1⁄2 miles (33.0 km)
• Swinton and Knottingley Joint Railway 19+1⁄2 miles (31.4 km)
• Tottenham & Hampstead Junction Railway 4+3⁄4 miles (7.6 km)

Joint with GWR

• Great Western and Great Central Joint Railway 41 miles (66 km)