| M & G Inc. | GMGX | added 1/2001; eliminated 4/2001 | |
| M & G Polymers U.S.A. LLC | GMGX | added 4/2001 | |
| M & M Chemical Products, Inc. | MMCX | added 7/77; 1/80; eliminated 7/89 | |
| M&B R.R., L.L.C. | MB | added 10/97; 7/2000 | |
| MNBR | added 10/97; 7/2000 | ||
| M&C Railcar Leasing, L.L.C. | JDSX | added 1/97; 7/2000 | |
| M. Corp. Management Solutions, Inc. | BRKX | added 10/89; eliminated 4/90 | |
| M. Steffen & Co., Inc. | STX | 1/52-4/61 | |
| M.B. Suydam Co. | MBSX | Apr-38 | |
| M.B. Suydam Division, Pittsburgh Plate Glass Co. | MBSX | 1/52; eliminated 4/54 | |
| M.B.C.X. Leasing, L.L.C. | MBCX | added 7/98; 7/2000 | |
| M4 Holdings Ltd. | MFFX | added 4/85; eliminated 1/92; 1/94-7/2000 | |
| Mac Acquisitions, L.P. | MACX | added 10/96; 7/2000 | |
| Mackinac Transportation Co. | MCTR | Jan-75 | |
| MacMillan-Bloedel Inc. | MBPX | added 4/73; 1/75-1/94; eliminated 1/98; 7/2000 | |
| MacMillan-Bloedel Products, Inc. | MBPX | added 10/71; eliminated 4/73 | |
| Macon Terminal Co. | MTCO | 1/75-7/2000 | |
| Macon, Dublin & Savannah R.R. Co. | MD&S | 4/38-10/57 | |
| Madison Coal Corp. | MCCX | eliminated 7/63 | |
| MCCX | Apr-61 | ||
| Madison R.R., Division of City of Madison Port Authority | CMPA | added 1/79; 1/80; eliminated 7/90; 1/94-7/2000 | |
| Madison Ry. Co., Inc. | MDRY | added 1/78; eliminated 1/79 | |
| Maersk, Inc. | MAEX | added 7/90; eliminated 4/93; 1/94 | |
| Magma Arizona R.R. Co. | MAA | 1/75-1/80; eliminated 1/94; 7/2000 | |
| Magma Metals Co. | PVCX | added 4/93; 1/94; eliminated 10/96 | |
| Magnetic Pigment Co. | MPX | Apr-38 | |
| Magnimet, Monroe Scrap Division (interplant service) | MAGX | 1/80-1/94 | |
| Magnolia Petroleum Co. | MPCX | 4/38-10/57; eliminated 1/60 | |
| Magor Car Corp. | MACX | 7/65-10/66; eliminated 10/67 | |
| Magor Railcar Division, Fruehauf Corp. | MACX | added 7/68; 4/70 | |
| Mahanoy and Hazleton Ry., Inc. | MHRR | eliminated 10/86 | |
| Maine Central R.R. Co. | MEC | 4/38-1/94; eliminated 4/97; added 7/98; 7/2000 | |
| Maine Coast R.R. Corp. | MC | added 10/92; eliminated 10/93; 1/94; eliminated 1/97; added 10/99; 7/2000 | |
| Maine Sugar Industries, Inc. | MSIX | Apr-70 | |
| Maintenance Inc. | MNCX | 10/57-4/61; eliminated 4/62 | |
| Mallard Transportation Co. | MTCX | 7/65-7/2000 | |
| Manchester Gas Co. | MANX | added 4/74; 1/75-1/80; eliminated 4/89 | |
| Mandak Metal Processors Division of Gerdau MRM Steel Inc. | MDKX | Jul-00 | |
| Mangels, Herold Co., Inc. | MHTX | 4/38-10/57 | |
| Mangum Oil & Gas Co. | MOGX | added 4/88; eliminated 7/93 | |
| Manistee & Northeastern Ry. Co. | M&NE | 4/38-1/52 | |
| Manistique & Lake Superior R.R. | MLS | 4/38-10/66 | |
| Manufacturers' Junction Ry. Co. | MJ | 4/38-7/2000 | |
| Manufacturers Leasing Corp. | DYLX | Apr-61 | |
| Manufacturers Leasing Corp. | DYLX | added 1/62; 7/65-1/80; eliminated 4/84 | |
| KEBX | added 4/62; 7/65-1/80; eliminated 4/84 | ||
| Manufacturers Ry. Co. | MRS | Apr-38 | |
| MRS | 10/57-7/2000 | ||
| Manufacturers Ry. Co. of St. Louis | MRS | Jan-52 | |
| Mapco Gas Products, Inc. | MAPX | eliminated 10/93 | |
| Mapco Petroleum, Inc. | MAPX | added 4/85 | |
| MAPX | added 10/93; 1/94; eliminated 10/99 | ||
| Mapco Products Co. | MAPX | added 10/80; eliminated 4/85 | |
| Maple Leaf Mills Ltd. | TORX | 7/65-1/75; eliminated 7/77 | |
| Marathon Ashland Petroleum LLC | AOIX | added 1/99; 7/2000 | |
| Marathon Corp. | MPMX | 1/52-10/57 | |
| Marathon Leasing Co. | RTMX | added 10/70 | |
| Marathon Paper Mills Co. | MPMX | Apr-38 | |
| Marathon, a Division of American Can Co. | MPMX | eliminated 7/59 | |
| Marcus Rail Transport, L.L.C. | MCRX | Jul-00 | |
| Marcus-Ruth-Jerome Co. | MRJX | Apr-38 | |
| Marden-Wild Corp. | MWKX | 1/52-4/61 | |
| Marfax Steel and Railway Equipment Co. | MFCX | added 10/67; 4/70; eliminated 7/70 | |
| Marhoefer Packing Co., Inc. | MPKX | 4/70; eliminated 7/73 | |
| MPLX | added 1/69 | ||
| Marianna & Blountstown R.R. | M&BT | Apr-38 | |
| Marianna & Blountstown R.R. Co. | MBT | added 11/73; 1/75-1/80 | |
| Marine Oil Transportation | MOT | 1/75-7/2000 | |
| Marinette, Tomahawk & Western R.R. Co. | MTW | 1/75-1/80; eliminated 4/92 | |
| Mark Twain Marine Industries, Inc. | LSSX | added 10/69; 4/70; eliminated 7/74 | |
| Markwest Hydrocarbon Partners, Ltd. | MWHX | added 1/93; 1/94-7/2000 | |
| Marquette & Huron Mountain R.R. Co., Inc. | MHCO | 1/75-1/80 | |
| Marquette Co. | MQCX | added 7/76; eliminated 1/79 | |
| Marquette Co., a subsidiary of Gulf & Western Industries, Inc. | MQCX | added 1/79; 1/80; eliminated 4/80 | |
| Mars Inc. | MWEX | Apr-38 | |
| TMX | Apr-38 | ||
| Marshall Railway Equipment Corp. | MREX | added 7/56; 10/57-1/80; eliminated 10/87 | |
| Martin Gas Sales, Inc. | MGSX | added 1/81; 1/94-7/2000; eliminated 4/2001 | |
| Martin Marietta Aluminum Inc. | MMAX | added 7/82; eliminated 1/85 | |
| Martin Marietta Cement, Division of Martin Marietta Corp. | MMSX | added 1/75; 1/75; eliminated 1/79 | |
| Martin Marietta Corp. | MMAX | added 4/85; 1/94; eliminated 7/97; 7/2000 | |
| MMBX | added 1/79; 1/80; eliminated 7/86 | ||
| MMSX | added 1/79; 1/80 | ||
| DRCX | added 1/79; 1/80; eliminated 7/86 | ||
| HCTX | added 4/90; 1/94; eliminated 7/96; 7/2000 | ||
| Martin Marietta Materials, Inc. | MWAX | added 7/97; 7/2000 | |
| Martin Oil Service, Inc. | MOSX | 1/52-10/66; eliminated 7/67 | |
| Martin-Marietta Cement, Division of Martin-Marietta Corp. | DPCX | added 10/72; 1/75; eliminated 1/79 | |
| DPLX | added 10/72; 1/75; eliminated 1/79 | ||
| DRCX | added 10/72; 1/75; eliminated 1/79 | ||
| Martin-Marietta Corp. | DPCX | added 1/79; 1/80 | |
| DPLX | added 1/79; 1/80 | ||
| Marubeni America Corp. | KATX | eliminated 10/87 | |
| Maryland & Pennsylvania R.R. | M&PA | 4/38-10/66 | |
| Maryland & Pennsylvania R.R. Co. | M&PA | Apr-70 | |
| MPA | 1/75-1/94; eliminated 4/2000 | ||
| Maryland and Delaware R.R. Co. | MDDE | added 1/78; 1/80; eliminated 10/87; 1/94-7/2000 | |
| Maryland Midland Ry., Inc. | MMID | added 1/81; added 4/81; 1/94-7/2000 | |
| Massachusetts Call/Volunteer Firefighters Association | MCVX | Jul-00 | |
| Massachusetts Central | MCER | 1/80-1/94 | |
| Massachusetts Central R.R. | MCER | added 4/95; 7/2000 | |
| Master Builders, Inc. | MMBX | added 10/87; 1/94; eliminated 10/95; added 10/96; eliminated 1/97; 7/2000 | |
| Master Lubricants Co. | MLX | 4/38-4/61; eliminated 7/63 | |
| Material Brokerage Co. | MBCX | added 10/96; eliminated 7/98 | |
| Mather Refrigerator Express (Mather Stock Car Co.) | MRRX | Apr-38 | |
| MUNX | Apr-38 | ||
| Mather Stock Car Co. | MARX | added 10/53; eliminated 7/55 | |
| MCAX | 4/38-1/52; eliminated 7/55 | ||
| MORX | 1/52; eliminated 7/55 | ||
| MRX | eliminated 7/55 | ||
| MSCX | 4/38-1/52; eliminated 7/55 | ||
| MTX | eliminated 7/55 | ||
| MUNX | Jan-52 | ||
| GSX | Jan-52 | ||
| HPAX | Jan-52 | ||
| RPRX | eliminated 7/55 | ||
| MRRX | 1/52; eliminated 7/55 | ||
| Mathieson Chemical Corp. | MAWX | Jan-52 | |
| SASX | Jan-52 | ||
| Matthieson & Hegeler Zinc Co. | MHZX | 4/38-1/52 | |
| Maumee & Western R.R. Corp. | MAW | added 7/98; 7/2000 | |
| Max Transportation Service | MTSX | 7/65-4/70; eliminated 7/73 | |
| Maxton Oil & Fertilizer Co. | MOFX | added 7/65; 7/65-1/80; eliminated 1/91; 1/94-7/2000 | |
| Maxx Leasing Co. | MAAX | added 1/81; eliminated 10/88 | |
| Maywood & Sugar Creek | MAYW | 1/80-7/2000 | |
| MBF Industries, Inc. | MBFX | added 7/79; 1/80; eliminated 7/91 | |
| McCall Farm Chemicals, Inc. | MCFX | eliminated 4/55 | |
| McCloud River R.R. Co. | MR | 4/38-1/80; eliminated 7/92 | |
| McCloud River Ry. Co. | MCR | 1/75-1/80 | |
| McCloud Ry. Co. | MCR | 1/94-7/2000 | |
| MR | added 7/92; 1/94-7/2000 | ||
| McColl-Frontenac Oil Co., Ltd. | MFLX | 4/38-1/52 | |
| McCook Rendering Co. | MCRX | 10/57; eliminated 1/58 | |
| McCook Rendering Plant | MCRX | Jan-52 | |
| McCormick, Ashland City & Nashville R.R. Inc. | MACO | added 10/88; eliminated 1/93 | |
| McDonnell Douglas Corp. | DCCX | added 10/70; 1/75-1/80; eliminated 4/89; added 4/90; 1/94-7/2000 | |
| McDonough Brothers, Inc. | MCDX | added 7/78; 1/80; eliminated 10/83 | |
| McGoldrick Lumber Co. | MGLX | Apr-38 | |
| McGraw-Edison Co. | MEPX | added 4/69; 4/70-1/80; eliminated 10/93; 1/94-7/2000 | |
| McKeesport Connecting R.R. Co. | MKC | 1/52-7/2000 | |
| McKenzie Tank Lines, Inc. | MTLX | eliminated 4/65; 7/65 | |
| McKesson & Robbins, Inc. | BPCX | eliminated 7/65 | |
| McLaughlin Line R.R. | MCLR | added 7/88; eliminated 1/92; 1/94-7/2000 | |
| McMaster Grain Co. | SSGX | added 5/83; eliminated 4/90 | |
| Mears/CPG, Inc. | MESX | added 7/98; 7/2000 | |
| Mechling Bros. Chemicals, a Division of General Chemical Co. | MBX | Apr-38 | |
| Mells Cargo Supply, Inc. | MCSX | Jan-94 | |
| Melwire Inc. | MELX | eliminated 4/91 | |
| Memphis Union Station Co. | MUSC | 1/75-7/2000 | |
| Mercantile Refrigerator Line (Merchants Despatch Transportation Corp.) | MERX | 1/52-10/57 | |
| Mercersburg Ry., Inc. | MERW | added 4/79; 1/80; added 7/80; eliminated 1/81 | |
| Merchants Despatch Transporation Corp. | HNZX | added 7/55; 10/57-4/70; eliminated 10/71 | |
| Merchants Despatch Transportation Co. | MDAX | added 1/62; 7/65-1/80 | |
| MDFX | added 1/62 | ||
| MDSX | added 7/55 | ||
| MDT | 4/38-1/80; eliminated 1/83 | ||
| MERX | 4/61-4/70; eliminated 1/73 | ||
| MIDX | 7/65-4/70; eliminated 1/73 | ||
| MLKX | 1/52-4/70; eliminated 10/70 | ||
| MRBX | added 10/53; 10/57-4/70; eliminated 7/70 | ||
| ERDX | 1/52-1/80; eliminated 10/84 | ||
| ESGX | added 4/57; 10/57-10/66; added 4/68; 4/70 | ||
| QMRX | 10/57-7/65; eliminated 7/66 | ||
| ABCX | added 7/66; 10/66-4/70; eliminated 1/73 | ||
| MDTX | added 7/89; 1/94; eliminated 7/99 | ||
| MFVX | added 4/65; 7/65-1/75; eliminated 4/76; 1/80 | ||
| MSGX | 10/57-7/65; eliminated 10/65 | ||
| APPX | 1/52-10/66; eliminated 7/67 | ||
| BVDX | 1/52-4/61 | ||
| CDCX | added 7/55; 10/57-10/66; eliminated 7/69 | ||
| DICX | 1/52-7/65 | ||
| DICX | Apr-70 | ||
| DICX | 1/75-1/80; eliminated 1/80 | ||
| DRYX | added 10/60; 4/61 | ||
| DUFX | 1/52; eliminated 4/54 | ||
| ESHX | 7/65; eliminated 7/65 | ||
| GFCX | added 10/60; 4/61-1/80; eliminated 1/82 | ||
| HRIX | 10/57-10/66; eliminated 10/69 | ||
| IBPX | 4/61-4/70; eliminated 1/73 | ||
| IMCX | added 4/62 | ||
| Merchants Despatch Transportation Corp. | NCGX | 7/65-10/66; eliminated 7/68 | |
| NCSX | 7/65-10/66; eliminated 7/67 | ||
| NDIX | added 7/56; 10/57; eliminated 7/60; 4/61 | ||
| NHIX | 7/65; eliminated 7/66 | ||
| NMCX | 7/65-10/66; eliminated 4/67; 4/70 | ||
| NPMX | Oct-57 | ||
| NRBX | 7/65-1/94; eliminated 7/99 | ||
| NRC | 7/65-1/80; eliminated 1/83 | ||
| NYMX | 10/57-1/75; eliminated 7/79 | ||
| NYRB | added 1/58; eliminated 7/59 | ||
| NYRX | added 1/59; 4/61-1/75; eliminated 7/79 | ||
| OHPX | 10/57-4/61 | ||
| PVYX | added 1/69; 4/70-1/80; eliminated 10/84 | ||
| RSGX | 10/57-1/75; eliminated 7/79 | ||
| SUCX | 10/57-4/70 | ||
| WMRX | 10/57-10/66; eliminated 10/69 | ||
| Merchants Investment Co. | MATX | added 4/75; 1/80; eliminated 1/89 | |
| CREX | 7/65-1/80; eliminated 1/89 | ||
| JTHX | added 7/84; eliminated 1/89 | ||
| Merco, A Joint Venture | MERX | 1/94; added 1/97; 7/2000 | |
| Meridian & Bigbee R.R. Co. | MB | 1/75-1/94; eliminated 10/97 | |
| MBRR | 1/75-1/80 | ||
| Meridian Southern Ry., L.L.C. | MDS | Jul-00 | |
| Merrimac Chemical Co. -- Monsanto Chemical Co., Merrimac Division | MCHX | Apr-38 | |
| Metal & Thermit Corp. | GDX | Apr-38 | |
| Metal-Matic, Inc. | MMIX | added 7/73; 1/75-1/80; eliminated 4/84 | |
| Metalurgica Mexicana Penoles, S.A. | PENX | 7/65-1/75; eliminated 7/76 | |
| Metro-North Commuter R.R. | MNCW | 1/94-7/2000 | |
| Metropolitan Sanitary District of Greater Chicago | CSDX | added 7/56; 10/57-1/80; eliminated 10/89 | |
| Metropolitan Water Reclamation District of Greater Chicago | CSDX | added 10/89; 1/94; eliminated 10/94; added 10/96; eliminated 1/97; 7/2000 | |
| Mexican Pacific R.R. Co., Inc. (Ferrocarril Mexicano del Pacifico, S.A.) | MDP | 1/75-1/80 | |
| Mexican Pacific R.R. Co., Inc. (transferred to Ferrocarriles Nacionales de Mexico) | MDP | eliminated 10/88 | |
| Mexican Petroleum Corp. | MPLX | 4/38-1/52 | |
| AMOX | Apr-38 | ||
| Mexicana de Cobre, S.A. de C.V. | MDCX | added 7/89; 1/94-7/2000 | |
| Mexico North Western | NODM | 1/75-1/80 | |
| Meyer Kornblum Refrigerator Car Co. | MKPX | Apr-38 | |
| MG Industries | THCX | Jul-00 | |
| MG Rail, Inc. | MGRI | 1/94-7/2000 | |
| MHC Inc., Wholly Owned Subsidiary of ConAgra, Inc. | CAAX | added 10/92; 1/94-7/2000 | |
| CAGX | 1/80-7/2000 | ||
| PBLX | added 4/90; 1/94; eliminated 1/2000; 7/2000 | ||
| CRMX | 1/94; eliminated 1/2000; 7/2000 | ||
| LMWX | added 7/92; 1/94-7/2000; eliminated 4/2001 | ||
| PVFX | added 1/83; 1/94; eliminated 10/96 | ||
| PVGX | added 1/83; 1/94; eliminated 10/96 | ||
| MHF Logistical Solutions Inc. | MHFX | added 10/98; 7/2000 | |
| Michael D. Boxberger | MBBX | added 1/80; 1/80; eliminated 1/81 | |
| Michels Pipeline Construction, Inc. | MPCX | added 4/91; 1/94-7/2000 | |
| Michigan Alkali Co. | MAX | Apr-38 | |
| Michigan Central R.R. | MC | Jan-75 | |
| Michigan Chemical Corp. | MCTX | 10/57-1/75 | |
| Michigan Northern Ry. Co., Inc. | MIGN | eliminated 1/79; 1/80; eliminated 4/89 | |
| Michigan Pickle Co. | MPKX | Jan-52 | |
| Michigan Products Dairy Co. (Merchants Despatch Transportation Corp.) | MPDX | Apr-38 | |
| Michigan Shore R.R., Inc. | MS | added 7/91; 1/94-7/2000 | |
| Michigan Southern R.R., Inc. | MSO | added 4/91; 1/94; eliminated 1/98; 7/2000 | |
| Michigan Sugar Co. | MSGX | Apr-38 | |
| Michigan-Wisconsin Transportation Co. | MWTT | added 4/85; eliminated 10/91; 1/94 | |
| Mid Atlantic R.R. Co., Inc. | MRR | added 7/88; 1/94; eliminated 4/96 | |
| Mid South Milling Co., Inc. | MSMX | added 4/81; eliminated 10/93; 1/94 | |
| Mid-Am Equipment, Inc. | JCLX | added 7/79; 1/80; eliminated 10/84; added 10/93; 1/94; eliminated 7/95; 7/2000 | |
| Mid-Am Equipment, Inc. | NECX | added 7/81; 1/94-7/2000 | |
| Mid-America Locomotive & Car Repair, Inc. | MALX | Jul-00 | |
| Mid-American Car & Equipment Co. | MATX | added 7/65; 7/65-1/75; eliminated 4/75 | |
| MidAmerican Energy Co. | MARX | added 10/98; 7/2000 | |
| MAXX | added 4/96; 7/2000 | ||
| MRDX | added 4/99; 7/2000 | ||
| IOPX | added 4/96; 7/2000 | ||
| IPSX | added 4/96; eliminated 4/99 | ||
| LGSX | added 4/96; 7/2000 | ||
| LRDX | added 4/96; 7/2000 | ||
| TRIX | added 4/96; eliminated 4/99 | ||
| UNSX | added 4/96; 7/2000 | ||
| WRDX | added 4/96; 7/2000 | ||
| Mid-Continent Petroleum Corp. | COSX | 4/38-1/52 | |
| OSKX | Jan-52 | ||
| Middle Fork | MF | 1/75-1/80 | |
| Middleboro Chemical Industries, Inc. | MDCX | 4/70; eliminated 1/72 | |
| Middletown & Hummelstown R.R. Co. | MIDH | 1/80-7/2000 | |
| Middletown & Unionville R.R. | MU | Apr-38 | |
| Middletown and New Jersey Ry. Co., Inc. | MNJ | 1/75; added 4/78; 1/80-7/2000 | |
| Midland | MLD | 1/75-1/80 | |
| Midland Continental R.R. | MICO | Oct-66 | |
| Midland Cooperative Wholesale, Cushing Refining Division | CRGX | 1/52; eliminated 4/54 | |
| Midland Cooperatives, Inc. | CRGX | eliminated 1/69 | |
| Midland Cooperatives, Inc., Cushing Refining Division | CRGX | added 4/54; 10/57-10/66 | |
| Midland Ry. Co. of Manitoba | MLD | 4/38-10/66 | |
| Midland Terminal Rwy. Co. | MTR | Apr-38 | |
| Midland Valley R.R. Co. | MV | 4/38-10/66 | |
| MidLouisiana Rail Corp. | MDR | added 1/88; 1/94; eliminated 7/94 | |
| NLG | added 1/88; 1/94; eliminated 7/94 | ||
| Mid-Michigan R.R., Inc. | MMRR | added 4/88; 1/94-7/2000 | |
| Mid-South Bulk Services, Inc. | MBSX | added 4/2000; 7/2000 | |
| MidSouth Rail Corp. | MSRC | added 7/86; 1/94; eliminated 7/94 | |
| Midvale-Heppenstall Co. | MHSX | added 10/65; 10/66-4/70; eliminated 1/71 | |
| Midway | MID | Jan-80 | |
| Midway R.R. Co. | MID | Jan-75 | |
| Midwest Bottle Gas Co. | TRUX | added 1/78; 1/80-7/2000 | |
| Midwest By-Products Co. | MWBX | added 7/74; 1/75-1/80; eliminated 7/87 | |
| Midwest By-Products Co., a Division of Gilco, Inc. | MWBX | added 7/87; 1/94; eliminated 10/95 | |
| Midwest Coal Handling, Inc. | MWCL | 1/94-7/2000 | |
| Mid-West Despatch Car Co. | MWDX | Apr-38 | |
| REPX | Apr-38 | ||
| Midwest Energy Service Co. | IPSX | added 5/83 | |
| Midwest Energy Services Co. | MIDX | added 4/89; 1/94 | |
| Midwest Generation LLC | CWEX | added 10/2000 | |
| Midwest Grain Products, Inc. | MWSX | added 1/86; 1/94; eliminated 10/94 | |
| Midwest Locomotive Leasing & Sales, Inc. | MWLX | Jul-00 | |
| Mid-West Mineral Products, Inc. | MWMX | added 7/69; 4/70; eliminated 10/71 | |
| Mid-West Mud Co., Inc. | MWMX | added 1/78; 1/80; eliminated 4/85 | |
| Midwest Power Systems | IOPX | added 4/93; 1/94; eliminated 4/96 | |
| IPSX | added 1/94; eliminated 4/96 | ||
| TRIX | added 1/94; eliminated 4/96 | ||
| UNSX | added 4/93; 1/94; eliminated 4/96 | ||
| MidWest Rail Holdings, Inc. | MWRX | Jul-00 | |
| Midwest Railcar Corp. | MWCX | added 4/2000; 7/2000 | |
| Midwest Solvents Co., Inc. | MWSX | 1/80; eliminated 1/86 | |
| Midwest Transportation & Development Corp. | MTDX | added 4/2001 | |
| Midwestern Farm Fertilizers, Inc. | MFFX | 4/61-7/65; eliminated 10/65 | |
| Milchem Inc. | MILX | added 4/82; eliminated 7/88 | |
| Mile-High Railcar Leasing, Inc. | MHRX | added 1/80; 1/80; eliminated 1/88 | |
| MRSX | added 10/83; eliminated 1/88 | ||
| MHRX | added 1/88; eliminated 1/93; eliminated 4/93; 1/94-7/2000 | ||
| Mile-High Railcar Services, Inc. | MRSX | added 1/88; eliminated 4/93; 1/94-7/2000 | |
| Miles Inc. | MLEX | Jul-00 | |
| Milk Products Refrigerater Line (Merchants Despatch Transportation Corp.) | MLKX | Apr-38 | |
| Millennium Inorganic Chemicals, Inc. | MQPX | added 7/2000; 7/2000 | |
| Millennium Petrochemicals, Inc. | MIPX | 7/2000; eliminated 4/2001 | |
| Miller & Hart, Inc. (General American Transportation Corp.) | MAHX | Apr-38 | |
| Miller Compressing Co. | GCOX | 7/65-4/70; eliminated 1/73 | |
| Milpark Drilling Fluids | MILX | added 7/88; eliminated 7/93 | |
| Milstead | MLST | 1/75-1/80 | |
| Milwaukee Solvay Coke Co. | MSKX | 1/52-4/61 | |
| Milwaukee Solvay Coke Co., a Division of Pickands Mather & Co. | MSKX | added 7/66; 10/66-4/70; eliminated 10/70 | |
| Milwaukee Solvay Coke Co., Inc. | MSKX | eliminated 7/65 | |
| Milwaukee Solvay Coke Division, Manganese Chemicals Corp. | MSKX | added 7/65; eliminated 7/66 | |
| Milwaukee Solvents & Chemicals Corp. | MISX | added 1/74; 1/75-7/2000 | |
| Milwaukee Vinegar Co. | MVX | 4/38-1/52; eliminated 4/57 | |
| Min-Ad, Inc. | HSWX | 4/70; added 7/70; eliminated 10/70 | |
| Mindis Recycling | MIRX | added 1/97 | |
| Miner Enterprises | WHMX | 1/94; added 1/97; eliminated 4/97; 7/2000 | |
| Minera Mexico Internacional, Inc. | MIMX | added 7/78; 1/80; eliminated 7/87; 1/94-7/2000 | |
| Mineral Point Zinc Co. | MPZX | Apr-38 | |
| Mineral Point Zinc Division, The New Jersey Zinc Co. | MPZX | 1/52-10/57; eliminated 10/58 | |
| Mineral Range R.R. Co. | MLR | 4/38-1/52 | |
| Mineral Range, Inc. | MRAX | Jul-00 | |
| Mineral Wells & Eastern Ry. Co. | MWRY | added 7/90; eliminated 4/93; 1/94 | |
| Miner-Hillard Milling Co. | MHMX | 7/65-10/66; eliminated 4/67 | |
| Minneapolis & St. Louis Ry. Co. (Chicago and North Western Ry. Co.) | MSTL | 7/65-10/66 | |
| Minneapolis Eastern Ry. Co. | MINE | 1/75-1/80 | |
| Minneapolis Industrial Ry. Co. | MIR | 4/61-10/66 & 1/75-1/80; eliminated 1/85 | |
| Minneapolis, Anoka and Cuyuna Range R.R. Co. | MA&C | Apr-38 | |
| MACR | Jan-75 | ||
| Minneapolis, Northfield and Southern Ry. | MNS | 1/52-10/66 & 1/75-1/80; eliminated 4/80 | |
| Minneapolis, Northfield and Southern Ry., Inc. | MNS | added 4/80; eliminated 4/86 | |
| Minneapolis, Red Lake and Manitoba Ry. Co. | MRLM | Apr-38 | |
| Minneapolis, St. Paul & Sault Ste. Marie Ry. Co. | SOO | 4/38-4/61 | |
| Minnesota & International Ry. Co. | MI | Apr-38 | |
| Minnesota Central R.R. Co. | MCTA | added 4/95; eliminated 4/97; 7/2000 | |
| Minnesota Commercial Ry. Co. | MNNR | added 4/87; 1/94-7/2000 | |
| Minnesota Corn Processors Bulk Products Division | DMIX | added 10/95; 7/2000 | |
| LSIX | added 7/2000; 7/2000 | ||
| SYRX | added 4/96; 7/2000 | ||
| Minnesota Corn Processors, Inc. | MNCX | added 4/85; 1/94-7/2000; eliminated 10/2000 | |
| DMIX | added 4/95; eliminated 7/95 | ||
| Minnesota Corn Processors, Inc. Fructose Division | SYRX | added 4/94; eliminated 4/96 | |
| Minnesota Dept. of Public Service Weights and Measures Division | MNWX | added 7/93; 1/94; eliminated 10/99; 7/2000 | |
| Minnesota Mining & Manufacturing d/b/a 3M | MMMX | Jul-00 | |
| Minnesota Mining and Manufacturing Co., Inc. | MINX | 1/52-10/66; eliminated 4/68 | |
| Minnesota Northern R.R., Inc. | MNN |