| Abbreviation | Definition |
|---|---|
| SDIV | San Diego and Imperial Valley Railroad |
| SDIY | San Diego and Imperial Valley Railroad |
| SDIY | San Diego and Imperial Valley Railroad Company |
| SDL | Special Duty Locomotive |
| SDNR | San Diego Northern Railway |
| SDO | Shilo Danieltown And Oakland |
| SDPX | South Dakota Soybean Processors |
| SDRX | Sounder Commuter Rail |
| SDSU | Soo Line Railroad (Canadia Pacific Railway) |
| SDSU | Soo Line Railroad Company |
| SE | Ferrocarriles Unidos del Sureste |
| SE | Semo Port Railroad Incorporated |
| SEAU | Sea- Land Service Incorporated |
| SEAU | Sea-Land Service Inc |
| SECX | General Electric Railcar Services Corporation |
| SECX | South-East Coal |
| SEFU | Seafreight Line |
| SEFX | North American Rails Leasing Corporation |
| SEGX | SEGX Railcar |
| SEI | Southeastern International Corporation |