| Abbreviation | Definition |
|---|---|
| IRSZ | Interpool/Rail Services Limited |
| IRTV | Inter-Rail Transportation, Inc. |
| IRTX | Inter-Rail Transport |
| IRWZ | Industrial Railways |
| IRYM | Illinois Railway Museum |
| ISCU | I. S. C. Chemicals Limited |
| ISCU | I.S.C. Chemicals |
| ISCX | Illinois Steel Company (ISC Inc.) |
| ISCX | ISC Incorporated |
| ISCZ | In-Terminal Services Corporation |
| ISG | ISG Railways |
| ISIZ | Intermodal Services |
| ISIZ | Intermodal Services Incorporated |
| ISLU | Islandia |
| ISLU | Islandia Limited |
| ISLZ | ITS Technologies & Logistics, L.L.C. |
| ISPX | ISPAT Inland, Inc. |
| ISPX | Western of North America |
| ISPZ | Isu Power Plant |
| ISR | Iowa Southern Railroad |