| Abbreviation | Definition |
|---|---|
| MACX | Mac Acquisitions, L. P. |
| MADU | Magnum Speditionsgelleshaft |
| MADU | Magnum Speditionsgelleshaft GMBH |
| MAEU | Maersk Lines |
| MAGX | Magnimet (Monroe Scrap Division) |
| MAI | Munich, Augsburg, and Ingolstadt |
| MAIX | Macon Iron and Paper Stock |
| MAJU | Malaysian International Shipping Corporation |
| MALX | Mid- America Locomotive and Car Repair, Incorporated |
| MALX | Mid-America Locomotive and Car Repair |
| MALZ | American President Lines |
| MALZ | American President Lines Incorporated |
| MANX | Gas Supply Incorporated |
| MANX | Manchester Gas |
| MAPX | Mapco Petroleum Incorporated |
| MAPX | MAPCO Products |
| MARS | Midwest Association of Rail Shippers |
| MART | Mid- Atlantic Rail Trail |
| MARX | MidAmerican Energy Company |
| MARX | MidAmerican Energy Corporation |