| Abbreviation | Definition |
|---|---|
| MDSZ | Medspan Shipping Service Incorporated |
| MDT | Merchants Despatch Transportation Corporation |
| MDTX | Merchants Despatch Refrigerator Line |
| MDTX | Merchants Despatch Transportation Company |
| MDW | Minnesota Dakota and Western Railway Company |
| MDW | Minnesota, Dakota and Western Railway |
| MDWU | Minnesota Dakota and Western Railway Company |
| MDWU | Minnesota, Dakota and Western Railway |
| MDWZ | Minnesota Dakota and Western Railway Company |
| MDWZ | Minnesota, Dakota and Western Railway |
| ME | Morristown and Erie Railway |
| ME | Morristown and Erie Railway Incorporated |
| MEC | Maine Central Railroad |
| MEC | Maine Central Railroad (later Guilford Transportation Industries, now Pan Am Railways) |
| MEC | Maine Central Railroad (Pan Am Railways) |
| MECX | Trinity Rail Management (Murray Energy Corp) |
| MECX | Trinity Rail Management Incorporated |
| MEFX | MexFresh |
| MEFZ | MexFresh |
| MELX | Melwire Inc. (Sunbelt Cement) |