| 1 | MR | Air Mauritanie | ICAO: MRT, Callsign: Mike Romeo, IATA Prefix: 174, Designator MR reinsated by IATA. Founded in September 1962. Ceased operations during 2007. | RIM | AVCODES |
| 2 | MR | Mongolian Airlines Group | ICAO: MML, Callsign: Trans Mongolia, IATA Prefix: 861, Name changed to Hunnu Air | MGL | AVCODES |
| 3 | MR | McCloud Ry. Co. | added 7/1992; 1/1994-7/2005 | | NAKINA |
| 4 | MR | McCloud River R.R. Co. | 8/1920-1/1980; eliminated 7/1992 | | NAKINA |
| 5 | MR | McCloud River R.R. Co. | 4/38-1/80; eliminated 7/92 | | NRHS |
| 6 | MR | McCloud Ry. Co. | added 7/92; 1/94-7/2000 | | NRHS |
| 7 | MR | *MAINE RAIL | | | PWRC |
| 8 | MR | +(MCR) MCCLOUD RAILWAY COMPANY [MCCLOUD RIVER RAILWAY] | | | PWRC |
| 9 | MR | McCloud River Railroad (McLeod Railway) | | | RSRV |
| 10 | MR | Mission Railroad Spec. | | | TRAFFIC |
| 11 | MR | Metroferro Roma (\"Met.Ro.\") | | IT | VKM |
| 12 | MRA | Martinaire | Callsign: Martex, ICAO callsign changed from Martinaire U S | USA | AVCODES |
| 13 | MRA | Martinaire | | | ICAO1 |
| 14 | MRAA | MARTINEZ RODRIGUEZ, AARON | | | SCAC4 |
| 15 | MRAC | MOORE & SON CO | | | SCAC1 |
| 16 | MRAD | MCRAE, F D TRUCKING INC | | | SCAC4 |
| 17 | MRAE | McRAE, BILL, TRUCKING | | | SCAC1 |
| 18 | MRAE | MATHESON, ROY AND ASSOCIATES INC | | | SCAC4 |
| 19 | MRAF | MORGAN, D W | | | SCAC4 |
| 20 | MRAG | MARQUEZ RODRIGUEZ, ROBERT | | | SCAC4 |
| 21 | MRAP | J. A. MIARA TRANS INC. | | | SCAC1 |
| 22 | MRAP | MIARA, J A TRANSPORTATION INC | | | SCAC4 |
| 23 | MRAT | MAR & ALI TRANSFER | | | SCAC4 |
| 24 | MRAV | MORELAND OIL CO | | | SCAC4 |
| 25 | MRAW | MARS TRANSPORTATION INC | | | SCAC4 |
| 26 | MRAX | Mineral Range, Inc. | 7/2000-7/2005 (not listed) | | NAKINA |
| 27 | MRAX | Mineral Range, Inc. | Jul-00 | | NRHS |
| 28 | MRAX | MINERAL RANGE, INC. | | | PWRC |
| 29 | MRAX | Mineral Range | | | RSRV |
| 30 | MRAX | MINERAL RANGE INC | | | SCAC4 |
| 31 | MRAY | MICHAUD & RAYMOND OIL INC | | | SCAC4 |
| 32 | MRAZ | Beneš-Mráz | | CS | TRAFFIC |
| 33 | MRB | Centre Pankh Kazakhstan | Callsign: Toma, | KZ | AVCODES |
| 34 | MRB | Mitteldeutsche Regiobahn | Leipzig (Veolia-Verkehr Regio Ost) | D | privat-bahn.de |
| 35 | MRB | Mitteldeutsche Regiobahn | | D | TRAFFIC |
| 36 | MRBC | M & R TRUCK BROKERS | | | SCAC4 |
| 37 | MRBD | MURRAY, D B LTD | | | SCAC4 |
| 38 | MRBE | MIRABELLI, P J ENTERPRISES INC | | | SCAC4 |
| 39 | MRBH | MCEVE | | | SCAC4 |
| 40 | MRBJ | MOST RELIABLE COURIER INC | | | SCAC4 |
| 41 | MRBL | M & R TRANSPORTATION LLC | | | SCAC4 |
| 42 | MRBM | M & R TRUCKING | | | SCAC4 |
| 43 | MRBN | MORENO BENITEZ, EDIEL | | | SCAC4 |
| 44 | MRBO | MRB TRANSPORT INC | | | SCAC4 |
| 45 | MRBQ | M C & S TRANSPORT LTD | | | SCAC4 |
| 46 | MRBR | MR BROTHERS INC | | | SCAC4 |
| 47 | MRBT | MIRABITO, JAMES & SONS | | | SCAC4 |
| 48 | MRBU | GERMAN AEROSPACE CENTER | | D | BIC |
| 49 | MRBU | DEUTSCHES ZENTRUM FUR LUFT UND RAUM | | | SCAC4 |
| 50 | MRBV | MACLEANS READY MIX CONCRETE LTD | | | SCAC4 |
| 51 | MRBW | MAGRABBIT DEDICATED FLEET LLC | | | SCAC4 |
| 52 | MRBX | Merchants Despatch Transportation Co. | added 10/1953; 7/1954-4/1970; eliminated 7/1970 | | NAKINA |
| 53 | MRBX | Merchants Despatch Transportation Co. | added 10/53; 10/57-4/70; eliminated 7/70 | | NRHS |
| 54 | MRBX | MERCHANTS DESPATCH TRANSPORTATION CORP. | | | PWRC |
| 55 | MRBY | MUEBLES RUSTICOS BERNAL | | | SCAC4 |
| 56 | MRC | Medical Air Rescue | Callsign: Rescue, | E | AVCODES |
| 57 | MRC | Airmarc Airlines, Inc. | Callsign: Airmarc, | USA | AVCODES |
| 58 | MRC | Martin Companies | Callsign: Martrans, | USA | AVCODES |
| 59 | MRC | Michigan R.R. Co. | 7/1923 | | NAKINA |
| 60 | MRC | *MICHIGAN RAILROAD CO. | | | PWRC |
| 61 | MRC | *MONROE RAILROAD COMPANY | | | PWRC |
| 62 | MRC | METRO CAB INC | | | SCAC4 |
| 63 | MRC | Manufacturers Ry. Co. of St. Louis | Real: MRS | USA | TRAFFIC |
| 64 | MRCA | MARCOS CALIDAD S A DE C V | | | SCAC4 |
| 65 | MRCD | MERCHANTS DELIVERY | | | SCAC1 |
| 66 | MRCD | MERCHANTS DELIVERY INC | | | SCAC4 |
| 67 | MRCE | Mitsui Rail Capital Europe B.V. | Amsterdam NL | D | privat-bahn.de |
| 68 | MRCE | MAURER CONSTRUCTION LTD | | | SCAC4 |
| 69 | MRCE | Mitsui Rail Company Europe | | NL | TRAFFIC |
| 70 | MRCEDL | MRCE Dispolok | | EU | TRAFFIC |
| 71 | MRCF | MORRISON CUSTOM FEEDS INC | | | SCAC4 |
| 72 | MRCG | MARROCO TRUCKING INC | | | SCAC4 |
| 73 | MRCI | MARTINEZ CRUZ, ISRAEL | | | SCAC4 |
| 74 | MRCJ | MARTIN, CHRIS (STERNDRIVE SPECIALIS | | | SCAC4 |
| 75 | MRCK | MRC TRUCKING LLC | | | SCAC4 |
| 76 | MRCM | MASON, R C MOVERS INC | | | SCAC4 |
| 77 | MRCN | MR TRUCKING | | | SCAC4 |
| 78 | MRCO | MADISON RAILWAY COMPANY | | | PWRC |
| 79 | MRCO | MARCOTTE DISPOSAL COMPANY | | | SCAC4 |
| 80 | MRCS | MORRISON RAIL CAR SERVICE OPERATION | | | PWRC |
| 81 | MRCT | MRC TRANSPORT INC | | | SCAC4 |
| 82 | MRCU | {LLC "EMRCC"};ZIPCODE=197046 | | RUS | BIC |
| 83 | MRCU | MERCATOR SHIPPING | | | SCAC1 |
| 84 | MRCW | MARINE CROSS COUNTRY INC | | | SCAC4 |
| 85 | MRCX | General Electric Rail Services Corp. | added 1/2000; 7/2000-7/2005 | | NAKINA |
| 86 | MRCX | General Electric Railcar Services Corp. | added 10/1992; 1/1994; eliminated 1/2000 | | NAKINA |
| 87 | MRCX | Itel Rail Corp. | added 4/1988; eliminated 10/1992 | | NAKINA |
| 88 | MRCX | Evans Railcar Leasing Co. | added 10/1980; eliminated 4/1988 | | NAKINA |
| 89 | MRCX | United States Railway Leasing Co. | added 4/1980; eliminated 10/1980 | | NAKINA |
| 90 | MRCX | Morrison Railway Supply Corp. | added 4/1969; 4/1970-1/1980; eliminated 4/1980 | | NAKINA |
| 91 | MRCX | Metals Reserve Co., care of Tin Processing Corp. | 1/1943 | | NAKINA |
| 92 | MRCX | Merck & Co., Inc. | 7/1932 | | NAKINA |
| 93 | MRCX | Morrison Railway Supply Corp. | added 4/69; 4/70-1/80; eliminated 4/80 | | NRHS |
| 94 | MRCX | United States Railway Leasing Co. | added 4/80; eliminated 10/80 | | NRHS |
| 95 | MRCX | Evans Railcar Leasing Co. | added 10/80; eliminated 4/88 | | NRHS |
| 96 | MRCX | Itel Rail Corp. | added 4/88; eliminated 10/92 | | NRHS |
| 97 | MRCX | General Electric Railcar Services Corp. | added 10/92; 1/94; eliminated 1/2000 | | NRHS |
| 98 | MRCX | General Electric Rail Services Corp. | added 1/2000; 7/2000 | | NRHS |
| 99 | MRCX | *MERCK & CO., INC. | | | PWRC |
| 100 | MRCX | *METALS RESERVE CO. (TIN PROCESSING CORP.) | | | PWRC |
