| 1 | M&BT | Marianna & Blountstown R.R. | 7/1923-1/1950 | | NAKINA |
| 2 | M&BT | Marianna & Blountstown R.R. | Apr-38 | | NRHS |
| 3 | M&BT | MARIANNA & BLOUNTSTOWN RAILROAD | | | PWRC |
| 4 | M&K | Morgantown & Kingwood R.R. (Baltimore & Ohio R.R.) | 7/1923 | | NAKINA |
| 5 | M&K | Morgantown & Kingwood R.R. | 8/1920 | | NAKINA |
| 6 | M&K | BALTIMORE & OHIO RAILROAD [MORGANTOWN & KINGWOOD RAILROAD] | | | PWRC |
| 7 | M&LS | Manistique & Lake Superior R.R. | 7/1930-7/1936 | | NAKINA |
| 8 | M&LS | MANISTIQUE & LAKE SUPERIOR RAILROAD | | | PWRC |
| 9 | M&M | Meridian and Memphis Ry. Co. (Gulf, Mobile and Northern R.R. Co.) | 8/1920-7/1923 | | NAKINA |
| 10 | M&M | GULF, MOBILE & NORTHERN RAILROAD CO. [MERIDIAN & MEMPHIS RWY. CO.] | | | PWRC |
| 11 | M&NA | Missouri & Northern Arkansas Ry. Co. | 7/1923-4/1938 | | NAKINA |
| 12 | M&NA | Missouri & Northern Arkansas R.R. | 8/1920 | | NAKINA |
| 13 | M&NA | Missouri & Northern Arkansas Ry. Co. | Apr-38 | | NRHS |
| 14 | M&NA | MISSOURI & NORTH ARKANSAS RWY. CO. | | | PWRC |
| 15 | M&NE | Manistee & Northeastern Ry. Co. | 7/1930-7/1954 | | NAKINA |
| 16 | M&NE | Manistee & Northeastern R.R. Co. | 8/1920-7/1923 | | NAKINA |
| 17 | M&NE | Manistee & Northeastern Ry. Co. | 4/38-1/52 | | NRHS |
| 18 | M&NE | MANISTEE & NORTHEASTERN RY. CO. | | | PWRC |
| 19 | M&PA | Maryland & Pennsylvania R.R. Co. | 4/1970 | | NAKINA |
| 20 | M&PA | Maryland & Pennsylvania R.R. (Co.) | 8/1920-10/1966 | | NAKINA |
| 21 | M&PA | Maryland & Pennsylvania R.R. | 4/38-10/66 | | NRHS |
| 22 | M&PA | Maryland & Pennsylvania R.R. Co. | Apr-70 | | NRHS |
| 23 | M&PA | MARYLAND & PENNSYLVANIA RAILROAD CO. | | | PWRC |
| 24 | M&RR | Minneapolis & Rainy River Ry. Co. | 8/1920-7/1932 | | NAKINA |
| 25 | M&RR | MINNEAPOLIS & RAINY RIVER RWY. CO. | | | PWRC |
| 26 | M&STL | MINNEAPOLIS & ST. LOUIS | | | PWRC |
| 27 | M&U | Middletown & Unionville R.R. | 8/1920-7/1936; 1/1941-1/1946 | | NAKINA |
| 28 | M&U | MIDDLETOWN & UNIONVILLE RAILROAD | | | PWRC |
| 29 | M&W | Morgantown & Wheeling Ry. Co. | 7/1923 | | NAKINA |
| 30 | M&W | MORGANTOWN & WHEELING RWY. CO. | | | PWRC |
| 31 | M&WR | Montpelier & Wells River R.R. | 7/1930-1/1941 | | NAKINA |
| 32 | M&WR | Montpelier & Wells River R.R. (Boston & Maine R.R.) | 8/1920-7/1923 | | NAKINA |
| 33 | M&WR | Montpelier and Wells River R.R. | Apr-38 | | NRHS |
| 34 | M&WR | BOSTON & MAINE RAILROAD [MOHTPELIER & WELLS RIVER RAILROAD] | | | PWRC |
