| Abbreviation | Definition |
|---|---|
| MDCX | Mexicana De Cobre de CV |
| MDCX | Mexicana De Cobre, S. A. de C. V. |
| MDDE | Maryland and Delaware Railroad |
| MDDX | Old Line Holding |
| MDDX | Old Line Holdings Company Incorporated |
| MDIX | Modern Dispersions |
| MDIX | Modern Dispersions Incorporated |
| MDKX | Gardau MRM Steel Mandak Metal Processors Division |
| MDLR | Midland Terminal |
| MDLR | Midland Terminal Company |
| MDM | Murray Design & Manufacturing Corporation Locomotive Works |
| MDN | Meriden Station (CT Rail - AMTK) |
| MDP | Mexican Pacific Railroad (Ferrocarril Mexicano del Pacifico) |
| MDPN | Train routing Midland, Michigan, to Port Huron, Michigan |
| MDR | Kansas City Southern Railway |
| MDR | Kansas City Southern Railway Company |
| MDRR | Miami Dade Rescue Railroad |
| MDS | Meridian Southern Railway |
| MDSB | BNSF Railway |
| MDSZ | Medspan Shipping Service |