| Abbreviation | Definition |
|---|---|
| SEIU | Strategic Equipment Systems |
| SEIU | Strategic Equipment Systems Incorporated |
| SEIX | Seimax Gas Corporation |
| SEIZ | Strategic Equipment Systems |
| SEIZ | Strategic Equipment Systems Incorporated |
| SEKR | Southeast Kansas Railroad |
| SEKR | Southeast Kansas Railroad Company |
| SELNEC | South East Lancashire North East Cheshire |
| SEMX | Seminole Electric Cooperative |
| SEPA | Southeastern Pennsylvania Transportation Authority |
| SEPA | Southeastern Pennsylvania Transportation Authority (SEPTA) |
| SEPX | Southwestern Electric Power |
| SEPX | Southwestern Electric Power Company |
| SEQU | Simmonds Equipment |
| SEQX | Sequa Chemicals |
| SEQX | Sequa Chemicals Incorporated |
| SERA | Sierra Railroad |
| SERA | Sierra Railroad Company |
| SERX | ACF Industries Incorporated |
| SERX | ACF Industries Shippers Car Line Division |