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| ELR | Route | Section | Remarks |
|---|---|---|---|
| AAV | Ascot and Ash Vale Line | Ascot Jn to Ash Vale Jn | |
| ABD | Aberdare Branch | Abercynon Jn to former Cwmbach New Jn | |
| ABE1 | Allington Jn to Barkston East Jn | ||
| ABW | Abbotswood Branch | Abbotswood Jn to Norton Jn | |
| ACD | Allington Chord | Allington East Jn to Allington North Jn | |
| ACP | Avenue Coke Plant | Clay Cross North Jn (Goods) to Avenue Coke Plant | Understood to be disused |
| ACR | Ashford ‘E’ Jn to Ramsgate (via Canterbury West) | ||
| ACW | Acton Branch | Acton Canal Wharf to Willesden | |
| AEG | Allerton Curve | Allerton East Jn to Garston Jn | |
| AFE | Shrewsbury Loop | Abbey Foregate Jn to English Bridge Jn | |
| AFL | Aintree Fork Line | Sefton Jn to Metal Box Siding | |
| AFR | Avonmouth and Filton Line | Stoke Gifford Jn to Bristol Bulk Handling Terminal | |
| AFS | Aviemore to Forres | Continues onto Strathspey preserved railway | |
| AGW | Angerstein Wharf Branch | Angerstein Jn to Angerstein Wharf | |
| AGY | Grimesthorpe Jn to Attercliffe Goods Yard | ||
| AHB | Ashburton Branch | ||
| AHG | Waterloo Curve | Nine Elms Jn to Linford Street Jn | |
| AHX | Hunts Cross Chord | Allerton Jn to Hunts Cross West Jn | |
| AIW | Alton Itchen Abbas and Winchester Line | ||
| AJB | Addlestone Jn to Byfleet Jn | ||
| AJM1 | Matlock Branch | Ambergate Jn to Matlock Riverside | Continues onto Peak Rail preserved railway |
| ALC1 | Aston and Litchfield Cross City Line (North) | Aston North Jn to Sutton Coldfield | |
| ALC2 | Aston and Litchfield Cross City Line (North) | Sutton Coldfield to Lichfield City Jn | |
| ALK | Aberdare Link Line / Branch | former Cwmbach New Jn to former Cwmbach Jn | |
| ALN1 | Haltwhistle to Alston | Understood to be disused | |
| AMB | Avonmouth (or Severn Beach) Branch | Severn Beach to St Andrew’s Jn | |
| AMJ | Ashton Moss Curve | Ashton Moss South Jn to Ashton Moss North Jn | |
| AML | Attenborough Jn to Meadow Lane Jn | ||
| ANI1 | Aberdeen to Inverness Line | Aberdeen to Keith Jn | |
| ANI2 | Aberdeen to Inverness Line | Keith Jn to Forres | |
| ANI3 | Aberdeen to Inverness Line | Forres to Milburn Jn | |
| ANL | Acton and Northolt Line | Old Oak Common West Jn to Northolt Jn | |
| ANN | Ayr and Mauchline Line | Newton Jn to Mauchline Jn (via Annbank Jn) | |
| ANS | Aldwarke North to Aldwarke South | ||
| APG | Aberpergwm Branch / No. 10 Railway | Understood to be disused | |
| APL | Dungeness Branch | Appledore Jn to Lydd Town | |
| APS | Aberthaw Power Station Branch | ||
| APY | Austin Pickersgills Yard | former Monkwearmouth Jn to Yard | Understood to be disused |
| AQL | Arcow Quarry Line | ||
| ARG1 | Argyle Line | Rutherglen Central Jn to Rutherglen North Jn | |
| ARG2 | Argyle Line | Strathclyde Jn to Finnieston East / West Jns | |
| ARH | Ardrossan Harbour Branch | Holm Jn to Ardrossan Harbour | |
| AST | Halliwell Branch | Astley Bridge Jn to Astley Bridge | Understood to be disused |
| ATG | Turnham Green to Gunnersbury Jn | ||
| ATH | Ashford ‘D’ Jn to Hastings | ||
| ATL | Atlantic Line | Peckham Rye to Battersea Park Jn | |
| AWL | Acton Wells Branch | Acton East to Acton Wells Jn | |
| AWP | Cottingham Branch | Anlaby Road Jn to West Parade North Jn | |
| AYH1 | Ayr Harbour Branch | Newton Jn to Ayr Harbour (NR boundary) | |
| AYH2 | Ayr Harbour Branch | Falkland Jn to Ayr Harbour Jn (via Falkland Yard) | |
| AYR1 | Glasgow to Ayr | Bridge Street Jn to Paisley Gilmour Street | |
| AYR2 | Glasgow to Ayr | Paisley Gilmour Street to Stoneybrae | |
| AYR3 | Glasgow to Ayr | Stoneybrae to former Dalry Jn | |
| AYR4 | Glasgow to Ayr | former Dalry Jn to Barassie Jn | |
| AYR5 | Glasgow to Ayr | Barassie Jn to former Lochgreen Jn | |
| AYR6 | Glasgow to Ayr | former Lochgreen Jn to Ayr | |
| AYS | Ashbury’s Yard Sidings |