| Abbreviation | Definition |
|---|---|
| SLUT | South Lake Union Transit |
| SLVX | Silver Enterprises |
| SLWC | Stillwater Central Railroad |
| SLWC | Stillwater Central Railroad Company Incorporated |
| SLWU | Saltworks Techologies |
| SLXU | Stewart Lubricants & Service Co |
| SLZU | CARU Specialized Leasing |
| SM | St. Mary's Railroad |
| SM | St. Marys Railroad Company |
| SMA | San Manuel Arizona Railroad |
| SMA | San Manuel Arizona Railroad Company |
| SMAR | Swindon Marlborough and Andover Railway |
| SMART | Sonoma Marin Area Rail Transit |
| SMCX | San Miguel Electric Cooperative |
| SMCZ | Maine Central Railroad |
| SMCZ | Maine Central Railroad Company |
| SMDU | Minnesota Mining and Manufacturing - 3M |
| SMDU | Minnesota Mining and Manufacturing (3M) |
| SMEX | South Mississippi Electric Power Association |
| SMIX | Structural Metals |