| Abbreviation | Definition |
|---|---|
| SOTX | Southern Concrete Construction Company Incorporated |
| SOU | Norfolk Southern Railway Company |
| SOU | Southern Railway (Norfolk Southern Railway) |
| SOUU | Norfolk Southern Railway Company |
| SOUU | Southern Railway (Norfolk Southern Railway) |
| SOUZ | Norfolk Southern Railway Company |
| SP | Southern Pacific |
| SP | Southern Pacific Railroad (Union Pacific Railroad) |
| SP | Union Pacific Railroad Company |
| SPAX | Southeastern Pennsylvania Transportation Authority |
| SPAX | Southeastern Pennsylvania Transportation Authority (SEPTA) |
| SPAZ | Spanish Lines |
| SPCH | Quest Communications Corporation |
| SPCRR | South Pacific Coast Rail Road |
| SPCU | Spacific Container Lease Pty |
| SPCU | Spacific Container Lease Pty Limited |
| SPCX | Qwest Communications Corporation |
| SPCX | Saskatchewan Power Corporation |
| SPDU | Southern Pacific Transportation (Spedycja Polska Spedcont) |
| SPDU | Union Pacific Railroad Company |