| Abbreviation | Definition |
|---|---|
| MBIX | Morse Brothers |
| MBKX | MRC Rail Services |
| MBLX | McColl Brothers Ltd. (Exxon-Mobil Corporation) |
| MBLX | Mobil Oil Corporation |
| MBPX | MacMillan-Bloedel Incorporated |
| MBPX | MacMillan-Bloedell |
| MBRL | Moreton Bay Rail Link |
| MBRR | Meridian and Bigbee Railroad |
| MBSX | Mid-South Bulk Services |
| MBT | Marianna and Blountstown Railroad |
| MBTA | Massachusetts Bay Transportation Authority |
| MBTX | Massachusetts Bay Transportation Authority |
| MC | Maine Coast Railroad Corporation |
| MC | Michigan Central Railroad (Massachusetts Coastal Railroad) |
| MCCX | Mount Vernon Coal Transfer |
| MCCX | Mt. Vernon Coal Transfer Company |
| MCDX | Redland Worth Corporation (Sunbelt Cement) |
| MCDX | Sunbelt Cement Incorporated |
| MCER | Massachusetts Central Railroad |
| MCER | Massachusetts Central Railroad Corporation |