| Name | Railroad, # | Builder, s/n | Build date | Whyte type | Notes |
|---|---|---|---|---|---|
| Mabel | LNWR 619 | Crewe | 2/1896 | 2-4-0 | Became LMS 5060; wd 5/1926. |
| MacCailin Mor | LNER 3445 | 2-6-0 | |||
| MacLeod of MacLeod | LNER 3446 | 2-6-0 | |||
| Madden | LMS 5668 | 4-6-0 | |||
| Madge | LNWR 2002 | Crewe | 6/1891 | 2-4-0 | Became LMS 5030; wd 12/1928. |
| Madge Wildfire | LNER 9244 | 4-4-0 | |||
| Madras | GWR 3407 | 4-4-0 | |||
| Madras | LMS 5575 | 4-6-0 | |||
| Madresfield Court | GWR 2947 | 4-6-0 | |||
| Magdala | LNWR 1744 | Crewe | 4/1892 | 2-4-0 | Wd 8/1915. |
| Magi | GWR | Gooch | 1856 | 0-6-0 | Wd 1879. |
| Magnificent | LNWR 1945 | 6/1901 | 4-4-0 | Rb 9/1915; became LMS 5139. | |
| Magpie | C&CR | 4-4-0T | |||
| Magpie | SDR | 4-4-0ST | |||
| Maid of Astolat | BR 73089 | Derby | 9/1955 | 4-6-0 | Wd 9/1966; scr 6/1967. |
| Maid of Astolat | SR 744 | 4-6-0 | |||
| Maid of Lorn | LNER 6396 | 4-4-0 | |||
| Maindy Hall | GWR 4942 | 4-6-0 | |||
| Majestic | GWR 3048 | GWR | 2/1895 | 4-2-2 | Wd 6/1913. |
| Malay States | LMS 5615 | 4-6-0 | |||
| Malcolm Graeme | LNER 6392 | 4-4-0 | |||
| Mallard | LNER 4468 | LNER Doncaster 1870 | 3/1938 | 4-6-2 | Certified as the record holder for speed by a steam locomotive, achieved 126 mph on July 3, 1938, operating on the East Coast Main Line. Preserved at the National Railway Museum. More information: Wikipedia. |
| Malmesbury Abbey | GWR 4062 | 4-6-0 | |||
| Malta | LMS 5616 | 4-6-0 | |||
| Malta | LNWR 2177 | 7/1911 | 4-4-0 | Became LMS 5358; wd 11/1936. | |
| Malvern | SR 929 | 4-4-0 | |||
| Mammoth | GWR | GWR | 1848 | 0-6-0 | Wd 1873. |
| Mammoth | LNWR 645 | Crewe 4461 | 1/1905 | 4-4-0 | Became LMS 5197; wd 11/1930. |
| Manchester City | LNER 2871 | 4-6-0 | |||
| Manchester United | LNER 2862 | 4-6-0 | |||
| Manitoba | LMS 5558 | 4-6-0 | |||
| Manna | LNER 2596 | 4-6-2 | |||
| Manorbier Castle | GWR 5005 | 4-6-0 | |||
| Manton Grange | GWR 6822 | 4-6-0 | |||
| Marathon | LNWR 517 | Crewe | 1/1896 | 2-4-0 | Wd 4/1921. |
| Marathon | LNWR 1542 | Crewe | 4-6-0 | Became LMS 5750; wd 3/1936. | |
| Marble Hall | GWR 5907 | 4-6-0 | |||
| Mark Twain | LNWR 86 | Crewe | 4-6-0 | Became LMS 5626; wd 5/1936. | |
| Marlas Grange | GWR 6841 | 4-6-0 | |||
| Marlborough | SR 922 | 4-4-0 | |||
| Marmion | LNWR 469 | Crewe 4665 | 6/1907 | 4-4-0 | Became LMS 5270; wd 5/1936. |
| Marne | LNER 5511 | 4-4-0 | |||
| Marquis | LNWR 412 | Crewe 4442 | 6/1904 | 4-4-0 | Became LMS 5188; rn 25188; wd 4/1940. |
| Marquis Douro | LNWR 124 | 2-4-0 | Became LMS 5090. | ||
| Marquis Wellington | 1814 | ||||
| Marrington Hall | GWR 4943 | 4-6-0 | |||
| Mars (1st) | GWR | Mather, Dixon 53 | 1840 | 2-2-2 | In service only 4/1840-12/1840. |
| Mars (2nd) | GWR | Longridge | 1841 | 2-2-2 | Wd 1868. |
| Mars | L&BR 10 | SR&Co | 1/1841 | 2-2-2 | Became Joint Committee #55; transferred to SER. |
| Mars | LMS 5698 | 4-6-0 | Became BR 45698. | ||
| Mars | LNWR 1977 | 7/1903 | 4-4-0 | Became LMS 5129; rb 2/1924. | |
| Martin | LNWR 793 | 2-4-0 | Became LMS 5101; rn Engineer Watford in 1923. | ||
| Marwell Hall | GWR 5946 | 4-6-0 | |||
| Maude | LNER 9673 | 0-6-0 | |||
| Mauritius | LMS 5617 | 4-6-0 | |||
| Mawley Hall | GWR 5959 | 4-6-0 | |||
| Mazeppa | CR | SSG | 1859 | 4-4-0ST | Transferred to SDR in 1866; became GWR 2111 in 1876. |
| Mazeppa | GWR | Rennie | 1841 | 2-2-2 | Wd 1868. |
| Medea | GWR | FM&J | 1842 | 2-2-2 | Wd 1873. |
| Medina | LBSCR 340 | 12/1881 | 2-2-2 | Wd 11/1908. | |
| Medina | SR 1 | 4-4-0 | Operated on the Isle of Wight. | ||
| Medusa | GWR | FM&J | 1842 | 2-2-2 | Wd 1864. |
| Medusa | LNWR 366 | Crewe 4481 | 5/1905 | 4-4-0 | Became LMS 5292; rn 25292; wd 4/1945. |
| Meg Dods | LNER 9410 | 4-4-0 | |||
| Meg Merrilees | LNER 9243 | 4-4-0 | |||
| Melisande | BR 73085 | Derby | 8/1955 | 4-6-0 | Wd 7/1967; scr 4/1968. |
| Melisande | SR 753 | 4-6-0 | |||
| Melling (1st) | GWR | SG&Co | 1865 | 2-4-0 | Wd 1877. |
| Melling (2nd) | GWR | GWR | 1877 | 2-4-0T | Wd 1892. |
| Melton | LNER 2543 | 4-6-2 | |||
| Melton Hall | LNER 2838 | 4-6-0 | |||
| Mentor | GWR | NG&Co | 1841 | 2-2-2 | Wd 1867. |
| Merchant Taylors | SR 910 | 4-4-0 | |||
| Mercury | BR 70020 | Crewe | 7/1951 | 4-6-2 | Wd 1/1967; scr 6/1967. |
| Mercury (1st) | GWR | Mather, Dixon 52 | 1838 | 2-2-2 | Wd 1843. |
| Mercury (2nd) | GWR | Longridge | 1841 | 2-2-2 | Wd 1865. |
| Mercury | GWR 3287 | 4-4-0 | |||
| Mercury | L&BR | SR&Co | 7/1841 | 2-2-2 | Became Joint Committee #57; transferred to SER. |
| Mercury | LNWR 749 | Crewe | 5/1895 | 2-4-0 | Became LMS 5052; wd 5/1927. |
| Mercury | SDR | Ince Forge | 1876 | 2-4-0ST | Ordered by SDR but not completed until after the GWR purchased the SDR; completed as GWR 1299 at Swindon; fitted with a crane in 1881; wd 1936. |
| Merevale Hall | GWR 5971 | 4-6-0 | |||
| Meridian | GWR | Hawthorn | 1840 | 2-2-2 | Wd 1870. |
| Merlin | BR 73080 | Derby | 6/1955 | 4-6-0 | Wd 12/1966; scr 5/1967. |
| Merlin | LNER 4486 | 4-6-2 | |||
| Merlin | LNWR 1978 | 7/1903 | 4-4-0 | Rb 12/1921; became LMS 5177. | |
| Merlin | SR 740 | 4-6-0 | |||
| Merrie Carlisle | LNWR 860 | Crewe | 11/1894 | 2-4-0 | Became LMS 5050; wd 6/1933. |
| Merry Hampton | LNER 2565 | 4-6-2 | |||
| Mersey | GWR | Gooch | 1857 | 0-6-0 | Wd 1879. |
| Mersey | LNWR 665 | Crewe 4664 | 6/1907 | 4-4-0 | Became LMS 5256; wd 10/1927. |
| Merstham | L&BR 3 | SR&Co | 7/1839 | 2-2-2 | Rn 20; became Joint Committee #52; transferred to LBSCR as #45; wd 10/1854. |
| Merstone | SR 27 | 0-4-4T | Operated on the Isle of Wight. | ||
| Messenger | LNWR 519 | Crewe 4482 | 5/1905 | 4-4-0 | Became LMS 5208; wd 12/1931. |
| Meteor | GWR | Hawthorn | 1840 | 2-2-2 | Wd 1864. |
| Meteor | LMS 5734 | 4-6-0 | |||
| Meteor | LNWR 2242 | 5/1913 | 4-4-0 | Became LMS 5398; wd 10/1936. | |
| Meteor | SDR | Longridge | 1851 | 4-4-0ST | Became GWR 2098 in 1876. |
| Metis | GWR | Gooch | 1855 | 0-6-0 | Wd 1877. |
| Midas | GWR | Gooch | 1854 | 0-6-0 | Wd 1875. |
| Middlesbrough | LNER 2855 | 4-6-0 | |||
| Middleton Hall | GWR 4944 | 4-6-0 | |||
| Milan | LBSCR 303 | Brighton | 1/1878 | 0-4-2 | Wd 6/1903. |
| Miles | GWR | GWR | 1866 | 0-6-0ST | Wd 1888. |
| Miles MacInnes | LNWR 2507 | 10/1910 | 4-4-0 | Became LMS 5335; wd 11/1935. | |
| Milligan Hall | GWR 4945 | 4-6-0 | |||
| Milo | GWR | NG&Co | 1841 | 2-2-2 | Wd 1866. |
| Milton | LNER 2835 | 4-6-0 | |||
| Milton | LNWR 1515 | Crewe | 5/1890 | 2-4-0 | Wd 11/1914. |
| Milton | LNWR 2055 | NBL | 4-6-0 | Became LMS 5678; wd 4/1935. | |
| Minerva | GWR | Gooch | 1853 | 0-6-0 | Wd 1877. |
| Minerva | LNWR 136 | NBL | 4-6-0 | Became LMS 5663; wd 1/1935. | |
| Minoru | LNER 2561 | 4-6-2 | |||
| Minos | GWR | FM&J | 1841 | 2-2-2 | Wd 1870. |
| Minotaur | LMS 5695 | 4-6-0 | |||
| Minotaur | LNWR 271 | Crewe | 6/1887 | 2-4-0 | Became LMS 5004; wd 12/1927. |
| Miranda | LNWR 1194 | Crewe | 2/1897 | 2-4-0 | Became LMS 5068; wd 6/1930. |
| Mogul | GWR | Kitson | 1862 | 2-4-0T | Rb as 2-4-0; wd 1872. |
| Mole | NDR | 2-2-2 | |||
| Monarch | GWR | Gooch | 1853 | 0-6-0 | Wd 1879. |
| Monarch | LNWR 419 | Crewe 4662 | 6/1907 | 4-4-0 | Became LMS 5255; wd 12/1930. |
| Monmouth Castle | GWR 5037 | 4-6-0 | |||
| Mons | LNER 5501 | 4-4-0 | |||
| Mons | LNER 9648 | 0-6-0 | |||
| Mons Meg | LNER 2004 | 2-8-2 | |||
| Montgomery Castle | GWR 5016 | 4-6-0 | |||
| Moonstone | LNWR 2583 | Crewe 4527 | 12/1905 | 4-4-0 | Became LMS 5235; wd 4/1935. |
| Moorhen | LNWR 1472 | 9/1911 | 4-4-0 | Became LMS 5371; rn 25371; wd 10/1939. | |
| Moose | GWR | GWR | 1846 | 2-2-2 | Wd 1870. |
| Moray Firth | BR 70053 | Crewe | 9/1954 | 4-6-2 | Wd 4/1967; scr 10/1967. |
| Morayshire | LNER 246 | 4-4-0 | |||
| Morden | LBSCR 35 | Brighton | 1878 | 0-6-0T | Rn 635; rn B635; rn 2635; rn 377S Loco Works Brighton; rn Brighton Works; rn DS377; rn 32635; rb 4/1922; wd 3/1963; scrapped. |
| Morecambe and Heysham | LMS 5526 | 4-6-0 | Became BR 45526. | ||
| Morehampton Grange | GWR 6853 | 4-6-0 | |||
| Moreton Hall | GWR 5908 | 4-6-0 | |||
| Morfa Grange | GWR 6866 | 4-6-0 | |||
| Morgan le Fay | BR 73112 | Doncaster | 10/1956 | 4-6-0 | Officially named 4/1960 at Eastleigh; wd 6/1965; scr 12/1965. |
| Morgan le Fay | SR 750 | 4-6-0 | |||
| Morlais Castle | GWR 5038 | 4-6-0 | |||
| Morning Star | BR 70021 | Crewe | 8/1951 | 4-6-2 | Wd 12/1967; scr 5/1968. |
| Morning Star | BR 92207 | Swindon | 6/1959 | 2-10-0 | Wd 12/1964; preserved at East Lancashire Railway. |
| Morning Star (1st) | GWR | R Stephenson | 1838 | 2-2-2 | Originally ordered and constructed for the New Orleans Railway; wd 1869. |
| Morning Star (2nd) | GWR 381 | Swindon | 1866 | 2-2-2 | Name removed circa 1897. |
| Morning Star | GWR 4004 | 4-6-0 | |||
| Moseley Hall | GWR 4946 | 4-6-0 | |||
| Mosquito | GWR | Vulcan | 1862 | 2-4-0T | Wd 1877. |
| Mostyn Hall | GWR 5985 | 4-6-0 | |||
| Mounts Bay | GWR 3273 | 4-4-0 | |||
| Munster | LMS 5740 | 4-6-0 | |||
| Murdoch | GWR | SG&Co | 1865 | 2-4-0 | Wd 1892. |
| Murdoch | LNWR 1488 | Crewe | 11/1888 | 2-4-0 | Became LMS 5014; wd 4/1932. |
| Mursley Hall | GWR 6915 | 4-6-0 | |||
| Myrtle | GWR | Swindon | 1864 | 2-4-0T | Also used as a tender locomotive; wd 1873. |
| Mysore | LMS 5586 | 4-6-0 | |||
| Mytton Hall | GWR 5996 | 4-6-0 |