| Abbreviation | Definition |
|---|---|
| ISR | Iowa Southern Railroad Company |
| ISRC | Idaho & Sedalia Transportation Company |
| ISRR | Indiana Southern Railroad |
| ISRR | Indiana Southern Railroad CompanyIncorporated |
| ISSR | ISS Rail |
| ISSR | ISS Rail Incorporated |
| ISSU | Iceland Steamship |
| ISSX | Interchange Specialty Service, Inc. |
| ISU | Iowa Southern Utilities (Southern Industrial Railroad) |
| ISUZ | Iowa Southern Utilities |
| ISW | Indiana Southwestern Railway |
| ISW | Indiana Southwestern Railway Company |
| ITA | Integrated transport authorities |
| ITAU | Italian Line |
| ITAU | Italian Lines |
| ITAX | Illinois Transit Assembly |
| ITAX | Illinois Transit Assembly Incorporated |
| ITB | Island Tug and Barge |
| ITC | Illinois Terminal Railroad (Norfolk Southern) |
| ITC | Norfolk Southern Railway Company (formerly Illinois Terminal Railroad) |