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| ELR | Route | Section | Remarks |
|---|---|---|---|
| MAC3 | Deepcar Jn to Cleethorpes | ||
| MAH | Harwich Town Branch | Manningtree South Jn to Harwich Town | |
| MAI | Maindee East Loop | Maindee North Jn to Maindee East Jn | |
| MAJ | Liverpool to Manchester | Hunts Cross West Jn to Manchester Central | |
| MAM1 | Maltby Colliery Branch Jn to ELR change | ||
| MAM2 | ELR change to Maltby Colliery | ||
| MAN1 | Manton Wood Colliery | Understood to be disused | |
| MAN2 | Manton Wood Colliery Branch | Understood to be disused | |
| MAS | Hope Valley (Dore and Chinley) Line | Totley Tunnel East to Chinley North Jn | |
| MAX | Maxwelltown Branch | Dumfries Station Jn to ICI Drungans Sidings | Understood to be disused |
| MBW1 | Esk Valley (Middlesbrough to Whitby) Line | Guisborough Jn to Battersby Jn | |
| MBW2 | Esk Valley (Middlesbrough to Whitby) Line | Battersby Jn to Grosmont Jn | |
| MBW3 | Esk Valley (Middlesbrough to Whitby) Line | Grosmont Jn to Whitby | |
| MCB | Markham Colliery Branch | Understood to be disused | |
| MCG | Maryport and Carlisle Goods Line | Carlisle Currock Jn to Rome Street Jn | |
| MCH | Macclesfield Branch | Cheadle Hulme Jn to Macclesfield | |
| MCJ1 | Marylebone to Claydon L&NE Jn Line | London Marylebone to Harrow-on-the-Hill South Jn | |
| MCJ2 | Marylebone to Claydon L&NE Jn Line | Harrow-on-the-Hill South Jn to former Quainton Road Jn | Understood to be part closed |
| MCJ3 | Marylebone to Claydon L&NE Jn Line | former Quainton Road Jn to former Calvert Jn | Understood to be part closed (adjacent to HS2 route) |
| MCJ4 | Marylebone to Claydon L&NE Jn Line | former Calvert Jn to former Claydon L&NE Jn | Understood to be disused |
| MCL | Moorgate and Midland City Line | Moorgate to Carlton Road Jn | |
| MCS | Margam Cripple and Holding Sidings | ||
| MDE | Mossend East Curve | Mossend South Jn to Mossend East Jn | |
| MDH1 | Penyfford West Chord | Connecting Line to Chester to Mold Branch | Understood to be disused |
| MDL1 | Manchester Diggle to Leeds | Thornhill LNW Jn to Copley Hill East Jn | |
| MDN | Mossend North Curve | Mossend North Jn to Mossend East Jn | |
| MDW | Mossend West Curve | Mossend South Jn to Mossend West Jn | |
| MDY | Maerdy Branch (Rhondda Fach) Branch | Understood to be disused | |
| MEB1 | Moorgate Branch | London Moorgate to Finsbury Park Jn | |
| MEN1 | Muirhouse South Jn to Eglinton Street Jn | Muirhouse Central Jn to Eglinton Street Jn | |
| MEN2 | Muirhouse South Jn to Eglinton Street Jn | Muirhouse South Jn to Muirhouse Central Jn | |
| MEW1 | Methley Jn to Whitwood Jn | Methley Jn to Methley Jn (for Whitwood) | Understood to be disused |
| MEW2 | Methley Jn to Whitwood Jn | Methley Jn (for Whitwood) to Whitwood Jn | |
| MGE | Milngavie Branch | Westerton Jn to Milngavie | |
| MGW | Milford Jn to Gascoigne Wood Jn | ||
| MGY | Middlesbrough Goods Yard | Understood to be part closed | |
| MHH | Heysham Branch | Morecombe Jn to Heysham Harbour | |
| MHL1 | Monktonhall Jn to Millerhill | former Wanton Walls Jn to Monktonhall Jn | |
| MHL2 | Monktonhall Jn to Millerhill | Millerhill East Jn to former Wanton Walls Jn | |
| MHL3 | Monktonhall Jn to Millerhill | Millerhill East Jn to Millerhill Yard | |
| MHL4 | Monktonhall Jn to Millerhill | Millerhill EMU Depot | |
| MHY | Niddrie Curve | Niddrie West Jn to Niddrie South Jn | |
| MIA | Manchester Airport Branch | Heald Green North Jn to Manchester Airport | |
| MII | Mistley Incline | Understood to be disused | |
| MIL | Milford Haven Branch | Johnston Jn to Milford Haven | |
| MIN | Minehead Branch | ||
| MIN1 | Fairwater High Output Yard Spur | ||
| MIR1 | Liverpool / Mersey Loop Line | Mann Island Jn to Mann Island Jn | |
| MIR2 | Liverpool / Mersey Loop Line | Mann Island Jn to Rock Ferry | |
| MIS | Southampton Western Docks Branch | Millbrook Jn to Southampton Western Docks | |
| MIT | Middleton Towers Branch | King’s Lynn to Middleton Towers (NR boundary) | |
| MJD | Marsh Jn Depot (Down Side) | ||
| MJI1 | Madeley Jn to Ironbridge Line | Madeley Jn to Lightmoor Jn | |
| MJI2 | Madeley Jn to Ironbridge Line | Ketley Jn to Lightmoor Jn to Ironbridge (NR boundary) | Understood to be part closed |
| MJS1 | Melbourne Jn to Sinfin | Melbourne Jn to Sinfin to Chellaston East Jn | Understood to be part closed |
| MJT1 | Mansfield Jn to Radford Jn | ||
| MJT2 | Radford and Trowell Line | Radford Jn to Trowell Jn | |
| MJU | St Philip’s Marsh Depot (Up Side) | ||
| MJW | Wimbledon Mitcham Line Jn to Mitcham North Jn | London Trams | |
| MKB1 | Monk Bretton Branch | Cudworth North Jn to ELR change | |
| MKB2 | Monk Bretton Branch | ELR change to Monk Bretton | |
| MLA | Anniesland Branch | Maryhill Park Jn to Anniesland | |
| MLE | Millerhill South Curve | Millerhill South Jn to Millerhill East Jn | Understood to be disused |
| MLG1 | Mallaig Branch | Fort William Jn to former Banavie Jn | |
| MLG2 | Mallaig Branch | former Banavie Jn to Mallaig | |
| MLN1 | Great Western Main Line (GWML) | London Paddington to Plymouth West | |
| MLN2 | Great Western Main Line (GWML) | Plymouth West to St Germans | |
| MLN3 | Great Western Main Line (GWML) | St Germans to Chacewater | |
| MLN4 | Great Western Main Line (GWML) | Chacewater to Penzance | |
| MLS | Margam Loco and Sorting Sidings | ||
| MMC | Markham Main Colliery Branch | Understood to be disused | |
| MOA | Mountain Ash Loop (or Deviation) | Mountain Ash Loop South Jn to Mountain Ash Loop North Jn | |
| MOB | Moretonhampstead Branch | Heathfield Branch Jn (Newton Abbot) to Heathfield | |
| MPC | Chessington Branch | Motspur Park Jn to Chessington South | |
| MPR1 | Miles Platting Station Jn to Rochdale East Jn | Miles Platting Station Jn to Thorpes Bridge Jn | |
| MPR2 | Miles Platting Station Jn to Rochdale East Jn (Hollinwood Branch) | Thorpes Bridge Jn to Oldham Werneth Jn | |
| MPR3 | Miles Platting Station Jn to Rochdale East Jn | Oldham Werneth Jn to Rochdale East Jn | |
| MRB | Milner Royd Jn to Bradford Interchange | ||
| MRH | Macclesfield, Bollington, and Marple Line | Macclesfield to Marple Wharf Jn | |
| MRJ | Margam Moors Junction Line | Margam Moors Jn to Margam Abbey Works East Jn | |
| MRL1 | Maryhill Line | Maryhill Park Jn to Cowlairs West Jn | |
| MRL2 | Maryhill Line | Maryhill Park Jn to Knightswood North Jn | |
| MSG2 | March to Spalding South Jn Line | Grassmoor Jn to Regional boundary | Understood to be disused |
| MSK | Misk Branch | Misk Branch Jn to ICI Misk | Understood to be part closed |
| MSM | Morecambe Branch | Morecambe South Jn to Morecambe | |
| MSW | Minster Loop | Minster West Jn to Minster South Jn | |
| MTE | Menstrie Branch | former Cambus Jn to Menstrie NR boundary | Understood to be disused |
| MTL1 | Methil (or Levenmouth) Branch | Thornton North Jn to Leven | Currently under reconstruction |
| MTL2 | Methil Branch | former East Fife Central Jn to Methil (NR boundary) | Understood to be part closed and partly replaced by MTL1 |
| MVE1 | Manchester Victoria to Euxton Jn | Manchester Victoria to Bolton | |
| MVE2 | Manchester Victoria to Euxton Jn | Bolton to Euxton Jn | |
| MVL1 | Manchester Victoria to Huddersfield | Miles Platting Jn to Stalybridge Jn | |
| MVL2 | Manchester Victoria to Huddersfield | Stalybridge Jn to Stalybridge | |
| MVL3 | Manchester Victoria to Huddersfield | Stalybridge to Heaton Lodge Jn | |
| MVL4 | Manchester Victoria to Huddersfield | Bradley Jn to Heaton Lodge East Jn | |
| MVM | Manchester Victoria to Miles Platting Jn | ||
| MVN1 | Manchester Victoria to Normanton | Manchester Victoria East Jn to Thorpes Bridge Jn | Manchester Metro |
| MVN2 | Manchester Victoria to Normanton | Thorpes Bridge Jn to Normanton | |
| MVR | Manifold Valley Light Railway | Understood to be disused | |
| MWB | Marlow Branch | Bourne End to Marlow | |
| MWC | Mouldsworth Jn to Helsby West Cheshire Jn | Understood to be disused | |
| MWJ | Marchey’s House Jn to Winning Colliery Crossing Jn | ||
| MWL | March West Curve | March West Jn to March Whitemoor Jn | |
| MWN | Grimsby Docks (or Great Coates) Branch | Marsh West Jn to Grimsby Docks | |
| MWY | Mwyndy Branch | former Mwyndy Jn to former Maesaraul Jn | Understood to be disused |
| MYC | Nuneaton North Chord | Midland Yard Jn to Canal Farm Jn |