| Abbreviation | Definition |
|---|---|
| MIRX | Mindis Recycling |
| MIRX | Regional Recycling |
| MIS | Mississippi Central Railroad |
| MIS | Mississippi Central Railroad Corporation |
| MISS | Mississippian Railway Cooperative |
| MISS | Mississippian Railway Cooperative Incorporated |
| MISU | Malaysian International Shipping Corporation |
| MISX | Milwaukee Solvents and Chemicals Corporation |
| MITA | Multilateral Interline Traffic Agreement |
| MJ | Manufacturers' Junction Railway |
| MJ | Manufacturers' Junction Railway Company |
| MJRX | M. Ryan Railway Service Contractors |
| MJVU | Merco |
| MJVU | Merco, A Joint Venture |
| MKA | Milwaukee Airport Amtrak Station |
| MKC | McKeesport Connecting Railroad |
| MKC | McKeesport Connecting Railroad Company |
| MKCX | Morrison Knudsen Corporation |
| MKE | Milwaukee Amtrak Station |
| MKFX | Century Rail Enterprises |