| 601 | MARX | *MISSISSIPPI ALABAMA RAILROAD AUTHORITY | | | PWRC |
| 602 | MARX | +MIDAMERICAN ENERGY COMPANY | | | PWRC |
| 603 | MARX | MidAmerican Energy Corporation | | | RSRV |
| 604 | MARY | MARYVILLE MARITIME INC | | | SCAC4 |
| 605 | MARZ | MARTRAC | | | PWRC |
| 606 | MARZ | MARTRAC | | | RSRV |
| 607 | MARZ | MARTRAC | | | SCAC4 |
| 608 | MAS | Malaysia Airlines System Berhad | IATA: MH, Callsign: Malaysian, IATA Prefix: 232, Name changed to Malaysia Airlines Berhad. Name changed from Malaysian Airlines Berhad (MAL) | MAL | AVCODES |
| 609 | MAS | Malaysia Airlines Berhad d/b/a Malaysia Airlines | IATA: MH, Callsign: Malaysian, IATA Prefix: 232, Name changed from Malaysia Airlines System Berhad. Name changed from Malaysian Airlines Berhad (MAL) | MAL | AVCODES |
| 610 | MAS | Malaysia Airlines | IATA: MH | | ICAO1 |
| 611 | MAS | Museumseisenbahn Ammerland-Satterland e.V. | Ocholt | D | privat-bahn.de |
| 612 | MAS | MARTIN SHAFFER | | | PWRC |
| 613 | MAS | MAS Special Engineering (BrightBus) | | GB | TRAFFIC |
| 614 | MASA | Malaysia Airlines (Penerbangan Malaysia) | Real: MAS | MAL | TRAFFIC |
| 615 | MASB | MITCHELL AND SONS TRUCKING | | | SCAC4 |
| 616 | MASE | MASTER AUTO TRANSPORT CORPORATION | | | SCAC4 |
| 617 | MASF | M A S LOGISTICS INC | | | SCAC4 |
| 618 | MASG | MASA TRUCKING CO | | | SCAC4 |
| 619 | MASI | METRO AIR SERVICES INC | | | SCAC4 |
| 620 | MASJ | MASTERSON, J CONSTRUCTION CORPORATI | | | SCAC4 |
| 621 | MaSJ | Marma - Sandarne Järnväg | 1858 - 1970 | S | svenska-lok.se |
| 622 | MASO | MILLER AND SONS TRUCKING LLC | | | SCAC4 |
| 623 | MASR | MASON, ROGER J LLC | | | SCAC4 |
| 624 | MASS | MASTER SHIPPNG LTD | | | SCAC4 |
| 625 | MASSBAY | Mass. Bay Transportation Authority | | USA | TRAFFIC |
| 626 | MAST | MARTIN, SAM TRUCKING INC | | | SCAC4 |
| 627 | MAST | Martin Schienentechnik KEG | | AT | VKM |
| 628 | MASU | MAS SHIPPING & TRADING | | ET | BIC |
| 629 | MASU | MAS LOGISTICS | | | SCAC4 |
| 630 | MASW | Mascot & Western R.R. Co. | 8/1920-7/1930 | | NAKINA |
| 631 | MASW | MASCOT & WESTERN RAILROAD CO. | | | PWRC |
| 632 | MASX | General Electric Railcar Services Corp. | added 1/1984; 1/1994; eliminated 10/1994 | | NAKINA |
| 633 | MASX | North American Car Corp. | added 4/1968; 4/1970-1/1980; eliminated 1/1984 | | NAKINA |
| 634 | MASX | North American Car Corp. | added 4/68; 4/70-1/80 | | NRHS |
| 635 | MASX | General Electric Railcar Services Corp. | 1/94; eliminated 10/94 | | NRHS |
| 636 | MASX | (NATX; NACC) GENERAL ELECTRIC RAILCAR SERVICES CORP. [MASONITE CORP. - LESSEE] | | | PWRC |
| 637 | MASX | GE Rail Services | | | RSRV |
| 638 | MASY | MARATHON PETROLEUM SUPPLY LLC | | | SCAC4 |
| 639 | MASZ | MAERSK SHIPPING | | | PWRC |
| 640 | MAT | Maine Aviation Corporation | Callsign: Maine-Av, | USA | AVCODES |
| 641 | MAT | Maine Air Transport, Inc. | Callsign: Maine Air, | USA | AVCODES |
| 642 | MAT | Maine Aviation | | | ICAO1 |
| 643 | MAT | Messer Austria GmbH | | AT | VKM |
| 644 | MATA | Memphis Area Transit Authority | City: Memphis | USA | TRAFFIC |
| 645 | MATB | MATTSON & SON HOUSEMOVING LTD | | | SCAC4 |
| 646 | MATE | MAT EXPRESS (DIVISION OF TRANSNAT E | | | SCAC4 |
| 647 | MATH | MARATHON LOGISTICS | | | SCAC4 |
| 648 | MATI | MATE TRUCKING INC | | | SCAC4 |
| 649 | MATICH | Matsue Ichibata Kotsu | | J | TRAFFIC |
| 650 | MATIS | MATISA Matériel Industriel SA | | CH | VKM |
| 651 | MATIT | Matisa S.p.A | | IT | VKM |
| 652 | MATJ | MERIDIAN AUTO TRANSPORT | | | SCAC4 |
| 653 | MATL | MID-AMERICAN TRUCK LINES INC | | | PWRC |
| 654 | MATL | MARTIN, LANDIS C TRUCKING | | | SCAC4 |
| 655 | MATM | MATMA ENTEPRISES LTD | | | SCAC4 |
| 656 | MATN | MAALT TRANSPORT | | | SCAC4 |
| 657 | MATO | MATEO TRUCKING | | | SCAC4 |
| 658 | MATR | MAYNARD TRANSPORTATION, INC | | | SCAC1 |
| 659 | MATR | Mátra Volán | | H | TRAFFIC |
| 660 | MATS | MATSON NAVIGATION CO. | | | PWRC |
| 661 | MATS | MATSON NAVIGATION CO. | | | SCAC1 |
| 662 | MATS | MATSON NAVIGATION CO | | | SCAC4 |
| 663 | MATS | Matsuura Railway | | J | TRAFFIC |
| 664 | MATSU | Matsumoto Electric Railway | | J | TRAFFIC |
| 665 | MATT | Mattagami Railroad | | USA | TRAFFIC |
| 666 | MATT | Matthäi Bauunternehmen | | DE | VKM |
| 667 | MATU | MATSON NAVIGATION COMPANY, INC | | USA | BIC |
| 668 | MATU | MATSON NAVIGATION CO. | | | PWRC |
| 669 | MATU | Matson Navigation | | | RSRV |
| 670 | MATU | MATSON NAVIGATION COMPANY INC | | | SCAC4 |
| 671 | MATV | MATS INC | | | SCAC4 |
| 672 | MATX | Railmark | added 1/1989 | | NAKINA |
| 673 | MATX | Merchants Investment Co. | added 4/1975; 1/1980; eliminated 1/1989 | | NAKINA |
| 674 | MATX | Mid-American Car & Equipment Co. | added 7/1965; 7/1965-1/1975; eliminated 4/1975 | | NAKINA |
| 675 | MATX | Mather Collieries | 7/1923-7/1930 | | NAKINA |
| 676 | MATX | Mid-American Car & Equipment Co. | added 7/65; 7/65-1/75; eliminated 4/75 | | NRHS |
| 677 | MATX | Merchants Investment Co. | added 4/75; 1/80; eliminated 1/89 | | NRHS |
| 678 | MATX | Railmark | added 1/89 | | NRHS |
| 679 | MATX | *MATHER COLLERIES | | | PWRC |
| 680 | MATX | *MARYLAND TRANSIT ADMINISTRATION | | | PWRC |
| 681 | MATX | *RAILMARK [MERCHANTS INVESTMENT CO.; MID-AMERICAN CAR & EQUIPMENT CO.] | | | PWRC |
| 682 | MATX | *MARYLAND TRANSIT ADMINISTRATION | | | PWRC |
| 683 | MATX | Merchants Investment | | | RSRV |
| 684 | MATY | MCTYRE TRUCKING COMPANY INC | | | SCAC4 |
| 685 | MATZ | MATSON NAVIGATION CO. | | | PWRC |
| 686 | MATZ | Matson Navigation | | | RSRV |
| 687 | MATZ | MATSON NAVIGATION CO. | | | SCAC4 |
| 688 | MAU | Air Mauritius | IATA: MK, Callsign: Airmauritius, IATA Prefix: 239, | MUS | AVCODES |
| 689 | MAU | Air Mauritius | IATA: MK | | ICAO1 |
| 690 | MAU | Mávaut | | H | TRAFFIC |
| 691 | MAUA | MAUI PARCEL SERVICE | | | SCAC4 |
| 692 | MAUB | MOLANDS AUTOBODY & TOWING | | | SCAC4 |
| 693 | MAUD | MICHAUD FURNITURE | | | SCAC4 |
| 694 | MAUJ | MAUST TRUCKING | | | SCAC4 |
| 695 | MAUK | MR A TRUCKING INC | | | SCAC4 |
| 696 | MAUN | MAUST CORPORATION | | | SCAC4 |
| 697 | MAUO | MAURICIOS TRUCKING | | | SCAC4 |
| 698 | MAUP | PORT OF MANATEE | | | PWRC |
| 699 | MAUQ | MASSMA | | | SCAC4 |
| 700 | MAUS | MAUI SODA & ICE WORKS LTD | | | SCAC4 |
