| 1 | M | Montour R.R. Co. | 7/1930-10/1966 | NAKINA | |
| 2 | M | Montour R.R. | 8/1920-7/1923 | NAKINA | |
| 3 | M | Montour R.R. Co. | 4/38-10/66 | NRHS | |
| 4 | M | MONTOUR RR | PWRC | ||
| 5 | M | Magyar Államvasutak | H | TRAFFIC | |
| 6 | M-K-T | Missouri-Kansas-Texas R.R. Co. | 4/1938 | NAKINA | |
| 7 | M-K-T | Missouri-Kansas-Texas R.R. Co. (M.-K.-T. Lines) | 7/1923-7/1936 | NAKINA | |
| 8 | M-K-T | Missouri-Kansas-Texas R.R. Co. | Apr-38 | NRHS | |
| 9 | M-K-T | MISSOURI, KANSAS & TEXAS RWY. (M.K. & T. LINES) | PWRC | ||
| 10 | M&BT | Marianna & Blountstown R.R. | 7/1923-1/1950 | NAKINA | |
| 11 | M&BT | Marianna & Blountstown R.R. | Apr-38 | NRHS | |
| 12 | M&BT | MARIANNA & BLOUNTSTOWN RAILROAD | PWRC | ||
| 13 | M&K | Morgantown & Kingwood R.R. (Baltimore & Ohio R.R.) | 7/1923 | NAKINA | |
| 14 | M&K | Morgantown & Kingwood R.R. | 8/1920 | NAKINA | |
| 15 | M&K | BALTIMORE & OHIO RAILROAD [MORGANTOWN & KINGWOOD RAILROAD] | PWRC | ||
| 16 | M&LS | Manistique & Lake Superior R.R. | 7/1930-7/1936 | NAKINA | |
| 17 | M&LS | MANISTIQUE & LAKE SUPERIOR RAILROAD | PWRC | ||
| 18 | M&M | Meridian and Memphis Ry. Co. (Gulf, Mobile and Northern R.R. Co.) | 8/1920-7/1923 | NAKINA | |
| 19 | M&M | GULF, MOBILE & NORTHERN RAILROAD CO. [MERIDIAN & MEMPHIS RWY. CO.] | PWRC | ||
| 20 | M&NA | Missouri & Northern Arkansas Ry. Co. | 7/1923-4/1938 | NAKINA | |
| 21 | M&NA | Missouri & Northern Arkansas R.R. | 8/1920 | NAKINA | |
| 22 | M&NA | Missouri & Northern Arkansas Ry. Co. | Apr-38 | NRHS | |
| 23 | M&NA | MISSOURI & NORTH ARKANSAS RWY. CO. | PWRC | ||
| 24 | M&NE | Manistee & Northeastern Ry. Co. | 7/1930-7/1954 | NAKINA | |
| 25 | M&NE | Manistee & Northeastern R.R. Co. | 8/1920-7/1923 | NAKINA | |
| 26 | M&NE | Manistee & Northeastern Ry. Co. | 4/38-1/52 | NRHS | |
| 27 | M&NE | MANISTEE & NORTHEASTERN RY. CO. | PWRC | ||
| 28 | M&PA | Maryland & Pennsylvania R.R. Co. | 4/1970 | NAKINA | |
| 29 | M&PA | Maryland & Pennsylvania R.R. (Co.) | 8/1920-10/1966 | NAKINA | |
| 30 | M&PA | Maryland & Pennsylvania R.R. | 4/38-10/66 | NRHS | |
| 31 | M&PA | Maryland & Pennsylvania R.R. Co. | Apr-70 | NRHS | |
| 32 | M&PA | MARYLAND & PENNSYLVANIA RAILROAD CO. | PWRC | ||
| 33 | M&RR | Minneapolis & Rainy River Ry. Co. | 8/1920-7/1932 | NAKINA | |
| 34 | M&RR | MINNEAPOLIS & RAINY RIVER RWY. CO. | PWRC | ||
| 35 | M&STL | MINNEAPOLIS & ST. LOUIS | PWRC | ||
| 36 | M&U | Middletown & Unionville R.R. | 8/1920-7/1936; 1/1941-1/1946 | NAKINA | |
| 37 | M&U | MIDDLETOWN & UNIONVILLE RAILROAD | PWRC | ||
| 38 | M&W | Morgantown & Wheeling Ry. Co. | 7/1923 | NAKINA | |
| 39 | M&W | MORGANTOWN & WHEELING RWY. CO. | PWRC | ||
| 40 | M&WR | Montpelier & Wells River R.R. | 7/1930-1/1941 | NAKINA | |
| 41 | M&WR | Montpelier & Wells River R.R. (Boston & Maine R.R.) | 8/1920-7/1923 | NAKINA | |
| 42 | M&WR | Montpelier and Wells River R.R. | Apr-38 | NRHS | |
| 43 | M&WR | BOSTON & MAINE RAILROAD [MOHTPELIER & WELLS RIVER RAILROAD] | PWRC | ||
| 44 | MA | Missouri and Arkansas Ry. | 1/1941-7/1954 | NAKINA | |
| 45 | MA | Missouri and Arkansas Ry. | Jan-52 | NRHS | |
| 46 | MA | MISSOURI & ARKANSAS RWY. | PWRC | ||
| 47 | MA | MID AMERICA CHARTER LINES INC | SCAC4 | ||
| 48 | MA&C | Minneapolis, Anoka and Cuyuna Range R.R. Co. | 7/1932-4/1938 | NAKINA | |
| 49 | MA&C | Minneapolis, Anoka and Cuyuna Range Ry. Co. | 7/1930 | NAKINA | |
| 50 | MA&C | Minneapolis, Anoka and Cuyuna Range Ry. | 7/1923 | NAKINA | |
| 51 | MA&C | Minneapolis, Anoka and Cuyuna Range R.R. Co. | Apr-38 | NRHS | |
| 52 | MA&C | MINNEAPOLIS, ANOKA & CUYUNA RANGE RAILROAD CO. | PWRC | ||
| 53 | MA&PA | MARYLAND & PENNSYLVANIA | PWRC | ||
| 54 | MA&S | Maxton, Alma & Southbound R.R. Co. | 8/1920-7/1923 | NAKINA | |
| 55 | MA&S | MAXTON, ALMA & SOUTHBOUND RAILROAD CO. | PWRC | ||
| 56 | MAA | MasAir | IATA: M7 | ICAO1 | |
| 57 | MAA | Magma Arizona R.R. Co. | 1/1975-1/1980; eliminated 1/1994; 1/1994 (not listed); 7/2000-7/2005 | NAKINA | |
| 58 | MAA | Magma Arizona R.R. Co. | 1/75-1/80; eliminated 1/94; 7/2000 | NRHS | |
| 59 | MAA | MAGMA ARIZONA RAILROAD COMPANY | PWRC | ||
| 60 | MAA | Magma Arizona Railroad | RSRV | ||
| 61 | MAA | Magma Arizona R.R. Co. | USA | TRAFFIC | |
| 62 | MAAA | MALISA TRANSPORTATION | SCAC4 | ||
| 63 | MAAB | M & M AMERICAN INC | SCAC4 | ||
| 64 | MAAC | MAAX BATH INC | SCAC4 | ||
| 65 | MAAF | MAASS FLANGE CORPORATION | SCAC4 | ||
| 66 | MAAG | MARS TRANSPORT INC | SCAC4 | ||
| 67 | MAAI | MARC G TRANSPORT INC | SCAC4 | ||
| 68 | MAAL | MARINE ACE CO LTD | SCAC4 | ||
| 69 | MAAM | MADRIGA | SCAC4 | ||
| 70 | MAAN | MARTAINER | SCAC4 | ||
| 71 | MAAO | MARTELL, VERNON | SCAC4 | ||
| 72 | MAAT | MORALES ARREOLA, ADOLFO | SCAC4 | ||
| 73 | MAAV | MARAN GAS MARITIME INC | SCAC4 | ||
| 74 | MAAW | MARQUEZ, ALBERTO TRANSPORT INC | SCAC4 | ||
| 75 | MAAX | Maxx Leasing Co. | added 1/1981; eliminated 10/1988 | NAKINA | |
| 76 | MAAX | Maxx Leasing Co. | added 1/81; eliminated 10/88 | NRHS | |
| 77 | MAAX | *MAXX LEASING CO. | PWRC | ||
| 78 | MAAX | *MEMPHIS AREA TRANSIT AUTHORITY | PWRC | ||
| 79 | MAAX | Maxx Leasing | RSRV | ||
| 80 | MAAY | NAAYKENS TRANSPORT LIMITED | SCAC3 | ||
| 81 | MAAY | MID AMERICAN TRUCKING INC | SCAC4 | ||
| 82 | MAB | Millardair | ICAO1 | ||
| 83 | MAB | MALVERN BRICK & TILE | PWRC | ||
| 84 | MAB | MOORE BESTWAY TRAVEL | SCAC4 | ||
| 85 | MABA | MARKALOO DESIGNS | SCAC4 | ||
| 86 | MABB | MID ATLANTIC SERVICES LLC | SCAC4 | ||
| 87 | MABD | MID ATLANTIC XPRESS INC | SCAC4 | ||
| 88 | MABE | MABE TRUCKING CO INC | SCAC4 | ||
| 89 | MABG | MAUD SHIPPING LTD | SCAC4 | ||
| 90 | MABH | MABRY, D R INC | SCAC4 | ||
| 91 | MABI | M & A TRUCKING BROKERAGE INC | SCAC4 | ||
| 92 | MABK | MASONBRINK TRUCKING | SCAC4 | ||
| 93 | MABN | M A B TRANSPORT | SCAC4 | ||
| 94 | MABS | MABEUS ENTERPRISES INC | SCAC4 | ||
| 95 | MABST | Manhattan & Bronx Surface Transit Operating Authority | USA | TRAFFIC | |
| 96 | MABSTOA | Manhattan & Bronx Surface Transit Operating Authority | USA | TRAFFIC | |
| 97 | MABU | MOHAMED ABDUL RAHMAN AL BAHAR | SCAC4 | ||
| 98 | MABX | MA BELL | PWRC | ||
| 99 | MABY | M & A TRANSPORT SERVICES INC | SCAC4 | ||
| 100 | MAC | Malta Air Charter | IATA: R5 | ICAO1 |