| Abbreviation | Definition |
|---|---|
| SOGX | Diamond Shamrock Chemicals |
| SOIX | Sand Seed Service |
| SOIX | Sand Seed Service Incorporated |
| SOL | Stratford Okahukura Line |
| SOLX | Solano Rail Car |
| SOM | Somerset Railroad |
| SOM | Somerset Railroad Corporation |
| SONX | Soneco Services |
| SONX | Soneco Services Incorporated |
| SOO | Soo Line Railroad (Canadian Pacific Railway) |
| SOO | Soo Line Railroad Company |
| SOOZ | Soo Line Railroad (Canadian Pacific Railway) |
| SOOZ | Soo Line Railroad Company |
| SOPR | Sandwich, Windsor, & Amherstburg Railway |
| SOPR | South Pierce Railroad |
| SOR | Southern Ontario Railway |
| SORA | Southern Rails Cooperative Limited |
| SORX | Sorrells Farm Supply |
| SOTP | Steam Over Tennessee Pass |
| SOTX | Southern Concrete Construction |