| Abbreviation | Definition |
|---|---|
| SMIX | Structural Metals Incorporated |
| SMLE | Society of Miniature Locomotive Engineers |
| SMLU | Seaboard Marine Limited |
| SMLZ | Seaboard Marine Limited |
| SMMX | St. Mary's Cement |
| SMMX | St. Marys Cement Company |
| SMNR | Southern Manitoba Railway |
| SMNX | Solvay Minerals |
| SMNX | Solvay Minerals Incorporated |
| SMPX | Commercial Metal Company |
| SMPX | Commercial Metals |
| SMR | South Maitland Railways |
| SMRF | Southeastern Model Railroad Forum |
| SMRR | Sisseton Milbank Railroad |
| SMRR | Sisseton Milbank Railroad Incorporated |
| SMRX | Atlanta Chapter B National Railway Historical Society Incorporated |
| SMRX | Southeastern Railway Museum |
| SMV | Santa Maria Valley Railroad |
| SMV | Santa Maria Valley Railroad Company |
| SN | Sacramento Northern Railway |