| Abbreviation | Definition |
|---|---|
| MRMZ | Monticello Railway Museum |
| MRR | Carolina Southern Railroad Company |
| MRR | Mid Atlantic Railroad (Carolina Southern Railroad) |
| MRRX | Murphy Road Recycling |
| MRS | Manufacturers Railway (Anheuser-Busch) |
| MRS | Manufacturers Railway Company |
| MRS | Military Road Switcher |
| MRS | Moron Rail Service |
| MRSAC | Modelling the Railways of South Australia Convention |
| MRSI | Minnesota Rail Service Improvement |
| MRSX | Mile-High Railcar Services |
| MRSX | Mile-High Railcar Services Incorporated |
| MRT | Mass Rail Transit |
| MRVC | Mumbai Rail Vikas Corporation |
| MRY | Maryn Rail Yard |
| MS | Michigan Shore Railroad |
| MS | Michigan Shore Railroad Incorporated |
| MSAZ | National Motor Freight Traffic Association |
| MSC | Mississippi Central (Illinois Central Gulf Railroad) |
| MSCI | Mississippi Central (Illinois Central Gulf Railroad) |