| Railroad | Records | Links |
|---|
| Totals | 26,537 | 978 |
|---|
| Macon and Birmingham Railway | 6 | |
| Macon and Brunswick Railroad | 11 | 1 |
| Macon and Western Railroad | 29 | 1 |
| Macon, Dublin and Savannah Railroad | 14 | |
| Mad River and Lake Erie Railroad | 21 | |
| Madison and Indianapolis Railroad | 6 | |
| Madison, Portage City and Lake Superior Railroad | 14 | |
| Mahanoy and Broad Mountain Railroad | 12 | 3 |
| Maine Central Railroad | 46 | 2 |
| Mammoth Cave Railroad | 2 | |
| Manassas Gap Railroad | 18 | |
| Manchester and Lawrence Railroad | 15 | 1 |
| Manchester and North Weare Railroad | 8 | |
| Manchester and Oneida Railway | 11 | |
| Manhattan Railway | 21 | 1 |
| Manistee and Grand Rapids Railroad | 12 | |
| Manistee and Luther Railroad | 6 | |
| Manistee and Northeastern Railroad | 20 | |
| Manistique Railway | 9 | |
| Manistique, Marquette and Northern Railroad | 6 | |
| Manitou and Pikes Peak Railway | 1 | |
| Mann's Creek Railroad | 2 | |
| Mansfield Railway and Transportation Company | 5 | |
| Manufacturers Railway | 5 | |
| Maricopa, Phoenix and Salt River Valley Railroad | 10 | |
| Marietta and Cincinnati Railroad | 45 | |
| Marietta and North Georgia Railway | 5 | |
| Marietta, Columbus and Cleveland Railroad | 9 | |
| Marion and Mississinewa Valley Railroad | 10 | |
| Marion and Rye Valley Railroad | 3 | |
| Marion Railroad | 16 | |
| Marquette and Ontonagon Railroad | 12 | |
| Marquette and Southeastern Railway | 6 | |
| Marquette, Houghton and Ontonagon Railroad | 5 | |
| Maryland and Delaware Railroad | | 2 |
| Maryland and Pennsylvania Railroad | 10 | |
| Mason and Oceana Railroad | 5 | |
| Mason City and Fort Dodge Railroad | 16 | |
| Mason-Hoge Railroad Construction Co. | | 2 |
| Massena Terminal Railroad | 6 | |
| Mattoon Railway | 4 | |
| Mauch Chunk and Switch Back Railroad | 7 | |
| McCauley Mountain and Black Creek Railroad | 13 | |
| McCloud River Railroad | 7 | |
| McKeesport Connecting Railroad | 13 | |
| McKeesport Terminal Railroad | 5 | |
| Medix Run Railroad | 4 | |
| Memphis and Charleston Railroad | 47 | 1 |
| Memphis and Little Rock Railroad | 17 | |
| Memphis and Ohio Railroad | 19 | |
| Mercer Valley Railroad | 2 | |
| Merrimack and Connecticut River Railroad | 13 | 1 |
| Metropolitan Railroad | 19 | 4 |
| Mexican Central Railway Company | 73 | 1 |
| Mexican International Railroad | 8 | |
| Mexican mineral Railway | 2 | |
| Mexican National Railroad | 29 | 1 |
| Mexican Northern Railway | 4 | |
| Mexico and Pacific Railroad | | 1 |
| Michigan and Ohio Railroad | 4 | |
| Michigan Central Railroad | 199 | 12 |
| Michigan Southern and Northern Indiana Railroad | 64 | 1 |
| Middleburg and Schoharie Railway | 13 | |
| Middletown and Cincinnati Railroad | 7 | 1 |
| Midville, Swainsboro and Red Bluff Railroad | 2 | |
| Mifflin and Centre County Railroad | 14 | |
| Mill Valley and Mount Tamalpais Scenic Railway | 5 | |
| Milledgeville and Eatonton Railroad | 8 | |
| Millen and Southwestern Railroad | 7 | |
| Miller County Railroad | 4 | |
| Milwaukee and Beloit Railroad | 21 | |
| Milwaukee and Horicon Railroad | 11 | |
| Milwaukee and Mississippi Railroad | 45 | |
| Milwaukee and Northern Railroad | 17 | |
| Milwaukee and Prairie du Chien Railway | 25 | |
| Milwaukee and St Paul Railway | 33 | 1 |
| Milwaukee and Watertown Railroad | 31 | 1 |
| Milwaukee, Benton Harbor and Columbus Railway | 6 | |
| Milwaukee, Lake Shore and Western Railway | 30 | 3 |
| Mine Hill and Schuylkill Haven Railroad | 19 | |
| Mineral Point Railroad | 20 | |
| Mineral Range Railroad | 17 | |
| Minneapolis and Pacific Railway | 5 | |
| Minneapolis and St Louis Railroad | 64 | 3 |
| Minneapolis Eastern Railway | 7 | |
| Minneapolis Sault Ste Marie and Atlantic Railway | 8 | |
| Minneapolis, St Paul and Ashland Railroad | 5 | |
| Minneapolis, St Paul and Sault St Marie Railway | 47 | 4 |
| Minnesota and International Railway | 10 | |
| Minnesota and North Wisconsin Railroad | 6 | |
| Minnesota and Northwestern Railroad | 13 | |
| Minnesota and Pacific Railroad | | 1 |
| Minnesota Central Railway | 18 | |
| Minnesota Transfer Railway | 6 | |
| Minnesota Valley Railroad | 22 | |
| Mississippi and Missouri Railroad | 25 | 2 |
| Mississippi and Tennessee Railroad | 19 | 1 |
| Mississippi Central Railroad | 20 | 1 |
| Mississippi River and Bonne Terre Railway | 9 | |
| Mississippi River, Hamburg and Western Railway | 6 | |
| Missouri Central Branch Railway | 4 | |
| Missouri Pacific Railway | 227 | 5 |
| Missouri River Fort Scott and Gulf Railroad | 5 | |
| Missouri Southern Railroad | 10 | |
| Missouri, Iowa and Nebraska Railroad | 4 | |
| Missouri, Kansas and Texas Railway | 109 | 6 |
| Mobile and Bay Shore Railway | 3 | |
| Mobile and Girard Railroad | 18 | |
| Mobile and Montgomery Railroad | 8 | 2 |
| Mobile and Ohio Railroad | 97 | 5 |
| Mobile, Jackson and Kansas City Railroad | 7 | |
| Moncton and Buctouche Railway | 7 | |
| Monongahela Connecting Railroad | 15 | |
| Monson Railroad | 9 | |
| Montana Central Railway | 11 | |
| Montana Railroad | 6 | |
| Montana Union Railway | 5 | |
| Monte Cristo Railway | 5 | |
| Montfort and Gatineau Colonization Railway | 6 | |
| Montour Railroad | 14 | |
| Montpelier and Wells River Railroad | 17 | |
| Morenci Southern Railway | 5 | |
| Morgantown and Kingwood Railroad | 3 | |
| Morris and Essex Railroad | 47 | 2 |
| Morris County Railroad | 1 | |
| Moscow, Camden and San Augustine Railway | 4 | |
| Moshassuck Valley Railroad | 3 | |
| Moss Point and Pascagoula Railroad | 4 | |
| Mount Airy and Eastern Railway | 2 | |
| Mount Carbon Railroad | 17 | |
| Mount Hope Mineral Railroad | 4 | |
| Mount Jewett, Kinzua and Riterville Railroad | 6 | |
| Mount Penn Gravity Railroad | 6 | |
| Mount Washington Railway | 7 | |
| Muncie Belt Railroad | 4 | |
| Muscatine North and South Railroad | 5 | |
| Muscogee and Georgia Railroad | 17 | |
| Muscogee Railroad | 15 | |
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