| IAIS | Iowa Interstate R.R., Ltd. | added 1/85; 1/94-7/2000 |
| IANR | Iowa Northern Ry. | added 1/82; 1/94-7/2000 |
| IAPX | Industrial Air Products Co. | added 10/65; 10/66-4/70 |
| Liquid Air, Inc. | 1/75-1/80; eliminated 1/87 | |
| Liquid Air Corp. | added 1/87; 1/94; eliminated 1/95 | |
| Air Liquide America Corp. | added 1/95; 7/2000 | |
| IAT | Iowa Terminal R.R. Co. | 1/75-1/80; added 10/80; eliminated 7/87 |
| Iowa Traction R.R. Co. | added 7/87; eliminated 1/88 | |
| IATR | Iowa Traction R.R. Co. | added 7/87; 1/94-7/2000 |
| IBCX | Indiana Boxcar Corp. | Jul-00 |
| IBPX | Merchants Despatch Transportation Corp. | 4/61-4/70; eliminated 1/73 |
| IBP, Inc. | added 5/83; 1/94; eliminated 7/95 | |
| Trinity Industries Leasing Co. | added 7/95; 7/2000 | |
| IBT | International Bridge & Terminal Co. | 1/75-1/94; added 7/98; 7/2000 |
| IC | Illinois Central R.R. Co. | 4/38-4/70 |
| Illinois Central Gulf R.R. Co. (Illinois Central) | 1/75-1/80 | |
| Illinois Central Gulf R.R. Co. | eliminated 4/88 | |
| Illinois Central R.R. Co. | added 4/88; 1/94-7/2000 | |
| ICCX | Inspiration Consolidated Copper Co. | added 7/74; 1/75; eliminated 10/78 |
| ICG | Illinois Central Gulf R.R. Co. | 1/75-1/80; eliminated 4/88 |
| Illinois Central R.R. Co. | added 4/88; 1/94-7/2000 | |
| ICIX | ICI America Inc. | added 4/72 |
| ICI United States Inc. | Jan-75 | |
| ICI Americas Inc. | 1/80-1/94; eliminated 4/96; 7/2000 | |
| ICLX | Intercoastal Leasing, Inc. | added 4/96; 7/2000 |
| ICMX | Illinois Cereal Mills, Inc. | added 4/92; 1/94-7/2000 |
| ICRK | Indian Creek R.R. Co. | eliminated 1/90; 1/94-7/2000 |
| Island Creek Coal Co. | added 1/79; 1/80; eliminated 7/89 | |
| Island Creek Corp. | added 7/89; 1/94-7/2000 | |
| IDAX | Industria del Alcali, S.A. | added 4/74; 1/75-1/94 |
| IDEX | ID Packing Company, Inc. | added 4/70; 4/70; eliminated 4/72 |
| Wilson-Sinclair Co. | eliminated 4/73 | |
| Wilson Car Lines, Division of Wilson & Co., Inc. | added 4/73; eliminated 7/73; 1/75-1/80 | |
| IDOX | EG&G Idaho, Inc. | added 4/87; 1/94; eliminated 4/95 |
| Lockheed Idaho Technologies Co. | added 4/95; 7/2000; eliminated 10/2000 | |
| U.S. Dept. of Energy, Idaho Operations Office | added 10/2000 | |
| IERX | General American Transportation Corp. | 1/52; eliminated 10/53 |
| IESX | Ivan E. Summa | added 4/74; 1/75-1/80; eliminated 7/81 |
| IESX Corp. | added 7/81; eliminated 10/87 | |
| Iowa Southern Utilities Co. | eliminated 1/96 | |
| IES Utilities, Inc. | added 1/96; eliminated 4/2000 | |
| Alliant Energy | added 4/2000; 7/2000 | |
| IFBX | Indiana Farm Bureau Cooperative Association, Inc. | added 7/79; 1/80; eliminated 4/90 |
| General Electric Railcar Services Corp. | added 4/90; 1/94; eliminated 1/2000 | |
| General Electric Rail Services Corp. | added 1/2000; 7/2000 | |
| IFCX | Interstate Foods Corp. | added 10/82; 1/94; eliminated 10/94; 7/2000 |
| IFOX | Industrial Fuel Oil, Inc. | added 1/81; eliminated 1/87 |
| Jeffrey J. Kelling | added 1/87; eliminated 7/89 | |
| IGN | International-Great Northern R.R. | 4/38-7/65 & 1/75-1/80 |
| IGPX | Industrial Steam Products | Jul-00 |
| IHB | Indiana Harbor Belt R.R. Co. | 1/75-7/2000 |
| IHCX | International Harvester Co. | 1/52-7/65; eliminated 10/65 |
| IHRC | Indiana Hi-Rail Corp. | added 4/82; 1/94; eliminated 1/95 |
| Wabash & Erie R.R. | added 1/95 | |
| Indiana Hi-Rail Corp. | added 4/95; eliminated 7/98; 7/2000 | |
| IHSX | Western Vancouver Island Heritage Society (Alberni Pacific Ry.) | Jul-00 |
| IICX | Interlake Iron Corp. (Toledo Furnace Plant) | Apr-38 |
| Interlake Iron Corp. | 1/52-4/61; eliminated 7/65 (reporting marks changed to ISCX) | |
| IIRC | Indiana Interstate Ry. Co., Inc. | added 4/78; 1/80; added 7/80; eliminated 1/83 |
| IISX | I.I. Selig & Sons Ltd. | added 10/68; 4/70; eliminated 10/70 |
| ILCX | International Latex Corp. | added 7/60; 4/61; added 7/64; 7/65-10/66; eliminated 10/68 |
| ILDX | Incandescent Lamp Department of General Electric Co. | Apr-38 |
| Lamp Division of General Electric Co. | 1/52-7/65 | |
| Incandescent Lamp Division of General Electric Co. | Oct-66 | |
| Lamp Division of General Electric Co. | Apr-70 | |
| General Electric Co., Lamp Business Division | 1/75; eliminated 1/78 | |
| General Electric Co., Lamp Business Group | added 1/78; eliminated 4/78 | |
| General Electric Co.--Lighting Business Group | added 4/78; 1/80-1/94; eliminated 7/97; 7/2000 | |
| ILSX | Independent Locomotive Service, Inc. | Jul-00 |
| ILW | Illinois Western R.R. Co. | Jul-00 |
| ILX | Inland Molasses Co. | Jan-52 |
| Great American Farm Implement Corp. | Oct-57 | |
| IMCX | Merchants Despatch Transportation Corp. | added 4/62 |
| International Minerals & Chemical Corp. | added 4/67; 4/70-1/80; eliminated 4/88 | |
| IMC Fertilizer, Inc. | added 4/88; 1/94; eliminated 1/95 | |
| IMC Global Operations, Inc. | added 1/95; 7/2000 | |
| IMEX | Indiana & Michigan Electric Co. | 1/52-4/61; added 10/70; 1/75-7/2000 |
| IMFX | General American Transportation Corp. | 1/75-1/80; eliminated 10/92 |
| IMID | Indiana Midland Ry. | added 4/85; eliminated 1/88 |
| IMMX | Industrial Minera Mexico, S.A. | added 7/75; 1/80; eliminated 5/83; 1/94-7/2000 |
| IMRL | I & M Rail Link, LLC | added 7/97; 7/2000 |
| IMRR | Illinois & Midland R.R. Inc. | added 7/96; 7/2000 |
| IMRX | Imperial Refineries | Apr-38 |
| Central States Oil Co. | 1/52; eliminated 7/55 | |
| Imperial Refineries Corp. | added 7/55; 10/57 | |
| IMSX | IMCO Services, a Division of Halliburton Co. | added 1/82; eliminated 1/86 |
| IMTX | Missouri Tank Car Corp. | Apr-38 |
| Multifoods Transportation, Inc. | added 7/80; eliminated 4/92 | |
| AGP, L.P. | added 4/92; 1/94; eliminated 10/95 | |
| Ag Processing, Inc. | added 10/95; 7/2000 | |
| IMWX | Transportation Management, Inc. | 1/75; added 4/75 |
| Missouri Valley Walnut Co. | 1/80; eliminated 1/82 | |
| IN | Illinois Northern R.R. Co. | 4/38-10/66 & 1/75; eliminated 4/75 |
| Indiana Northwestern R.R. Co. | added 4/93; eliminated 7/93 | |
| Indiana Northeastern R.R. Co. | added 7/93; 1/94-7/2000 | |
| INCX | Inco Ltd. | added 4/90; 1/94-7/2000 |
| INDX | Industrial Oil Corp. | Apr-38 |
| Industrial Propane Supply, Inc. | 7/65-4/70; eliminated 7/74 | |
| Indelpro, S.A. de C.V. | Jul-00 | |
| INFX | Infineum USA L.P. | added 1/2000; 7/2000 |
| INGX | Ingomar Packing Co. | 1/94; eliminated 10/94 |
| General Electric Railcar Services Corp. | added 10/94; eliminated 1/2000 | |
| General Electric Rail Services Corp. | added 1/2000; 7/2000 | |
| INLX | Inland Steel Co. | added 7/68; 4/70-1/75 |
| Inland Steel Coal Co. | Jan-80 | |
| Inland Steel Co. | eliminated 10/88 | |
| Incoal Co. | added 10/88; 1/94-7/2000 | |
| INOH | The Indiana & Ohio R.R., Inc. | 1/80; added 10/80 |
| The Indiana & Ohio Rail Corp. | eliminated 7/89 | |
| The Indiana & Ohio Rail System | added 7/89; added 4/93 | |
| The Indiana & Ohio R.R., Inc. | 1/94-7/2000 | |
| INOR | The Indiana & Ohio Rail System | eliminated 7/94; eliminated 10/98 |
| INOX | Indiana & Ohio Rail Corp. | added 1/95; 7/2000 |
| INPR | Idaho Northern & Pacific R.R. Co. | Jul-00 |
| INRD | The Indiana Rail Road Co. | added 4/86; 1/94; eliminated 7/99; 7/2000 |
| INT | Interstate R.R. | 4/38-10/66 & 1/75-1/80 |
| Southern Ry. System | eliminated 1/91 | |
| Norfolk Southern Ry. Co. | added 1/91; 1/94-7/2000 | |
| IOCR | The Indiana & Ohio Central R.R., Inc. | added 1/88; 1/94 |
| The Indiana & Ohio Rail System | eliminated 7/99 | |
| The Indiana & Ohio Central R.R. | added 7/99; 7/2000 | |
| IOCX | Illinois Oil Co. | Apr-38 |
| Rail Enterprises, Inc. | added 10/97; eliminated 4/98; 7/2000 | |
| IOER | The Indiana & Ohio Eastern R.R., Inc. | added 1/88; 1/94 |
| Indiana & Ohio Rail System | eliminated 10/94 | |
| IOPX | Iowa Power & Light Co. | 1/80; eliminated 4/93 |
| Midwest Power Systems | added 4/93; 1/94; eliminated 4/96 | |
| MidAmerican Energy Co. | added 4/96; 7/2000 | |
| IORY | The Indiana & Ohio Ry. Co. | added 1/88; 1/94 |
| The Indiana & Ohio Rail System | eliminated 7/99 | |
| The Indiana & Ohio Ry. | added 7/99; 7/2000 | |
| IOX | Imperial Oil, Ltd. | Apr-38 |
| Transit Co., Ltd. | 1/52; eliminated 10/53 | |
| Union Tank Car Co. | added 10/53; eliminated 10/57 | |
| Transit Co., Ltd. | added 10/57; 10/57-1/80 | |
| IPCX | International Paper Co. | 4/38-1/80; eliminated 10/81; added 10/84; 1/94-7/2000 |
| IPEX | Indeck Power Equipment, Co. | added 4/97; 7/2000 |
| IPLX | Illinois Power Co. | added 1/65; 7/65-1/80; eliminated 4/89; 1/94-7/2000 |
| IPOX | A & R Leasing, L.L.C. | added 7/97; 7/2000 |
| IPPX | Intermountain Power Agency | added 1/85; eliminated 4/85; added 1/86; 1/94; eliminated 7/99; 7/2000 |
| IPSX | Energy Development Co. | 1/75-1/80; eliminated 5/83 |
| Midwest Energy Service Co. | added 5/83 | |
| Midwest Power Systems | added 1/94; eliminated 4/96 | |
| MidAmerican Energy Co. | added 4/96; eliminated 4/99 | |
| GATX Capital Corp. | added 4/99; 7/2000 | |
| IPWX | Interstate Power Co. | added 7/79; 1/80-1/94; eliminated 4/2000 |
| IPWX | Alliant Energy | added 4/2000; 7/2000 |
| IQIX | Industria Quimica Del Istmo S.A. de C.V. | added 7/2000; 7/2000 |
| IR | Illinois RailNet, Inc. | Jul-00 |
| IRC | Canadian Government Rys. (Canadian National Rys.) | 23-Jul |
| IRCX | International Railway Car & Equipment Co. | Jan-52 |
| International Railway Car Co. | 10/57; eliminated 1/59 | |
| International Railway Car Leasing Co. | added 1/59; 4/61; eliminated 1/62 | |
| Morrison Plan, Inc. | added 1/62; 7/65; eliminated 4/66 | |
| Nationwide Railroad Leasing, Inc. | added 4/66; 10/66-4/70; eliminated 1/71 | |
| Island Creek Corp. | eliminated 7/97 | |
| IREX | IRECO, Inc. | added 10/86; eliminated 1/94; 7/2000 |
| IRN | Ironton R.R. Co. | 4/38-10/66 & 1/75; eliminated 4/76 |
| Consolidated Rail Corp. | 1/80-1/94 | |
| IRRC | Iowa R.R. Co. | added 1/82; eliminated 4/88 |
| IRRS | International Rail Road Systems, Inc. | Jul-00 |
| IRX | Wasatch Oil Co., Idaho Division | Jan-52 |
| ISCX | Illinois Steel Co. (Carnegie-Illinois Steel Corp.) | Apr-38 |
| Imperial Sugar Co. | added 7/60 | |
| International Sugar Co. | eliminated 10/60 | |
| Interlake Steel Corp. | added 7/65 (reporting marks changed from IICX); 7/65-10/66; eliminated 10/67 | |
| Interlake, Inc. | added 4/71; 1/75; eliminated 7/76 | |
| ISC, Inc. | 1/94-7/2000 | |
| ISN | Iowa Southern Utilities Co. | Apr-38 |
| ISPX | Iron & Steel Products Inc. | Jan-52 |
| United States Silica Corp. | added 10/78; 1/80; eliminated 1/80 | |
| The Western Company of North America | added 4/80; eliminated 7/92; 1/94 | |
| ISR | Iowa Southern R.R. Co. | added 10/87; eliminated 4/90; 1/94-7/2000 |
| ISRR | Indiana Southern R.R. Co., Inc. | added 10/92; eliminated 10/93; 1/94; added 7/96; 7/2000 |
| ISSR | ISS Rail, Inc. | added 10/93; 1/94-7/2000 |
| ISTX | Interstate Tank Car Corp. | 4/38-10/57; eliminated 10/59 |
| ISTX | North American Car Corp. | added 10/59; 4/61-1/80 |
| ISU | Iowa Southern Utilities Co. | Jan-52 |
| Southern Iowa Ry. Co. | 10/57-4/61 | |
| Southern Industrial R.R., Inc. | 7/65-10/66 | |
| Iowa Southern Utilities (Southern Industrial R.R., Inc.) | 1/75-1/80 | |
| ISW | Indiana Southwestern Ry. Co. | Jul-00 |
| ITAX | Illinois Transit Assembly Corp. | Jul-00 |
| ITB | Island Tug & Barge | 1/75-7/2000 |
| ITC | Illinois Terminal R.R. Co. | 1/52-10/66 & 1/75-1/80; eliminated 4/82 |
| Norfolk and Western Ry. Co. | added 7/82; eliminated 1/91 | |
| Norfolk Southern Ry. Co. | added 1/91; 1/94-7/2000 | |
| ITCX | Interstate Transport Co. | 4/38-4/61 |
| International Technologies Corp. | Jul-00 | |
| ITDX | SULCOM, Inc. | added 10/97; 7/2000 |
| ITGX | ITG Inc. | eliminated 1/87; 1/94-7/2000; eliminated 7/2001 |
| ITIX | Intermodal Technologies Inc. | 1/94-7/2000 |
| ITLX | Illinois Tank Line | Apr-38 |
| International Transport Co. | added 1/74; 1/75; eliminated 4/79 | |
| Itel Rail Corp. | eliminated 10/92 | |
| General Electric Railcar Services Corp. | added 10/92; 1/94; eliminated 1/2000 | |
| General Electric Rail Services Corp. | added 1/2000; 7/2000 | |
| ITTX | Trailer Train Co. (FMS flats specifically equipped to carry trucks (saddle back)) | added 7/65; 7/65-10/68 |
| Trailer Train Co. | Apr-70 | |
| Trailer Train Co. (flat cars equipped with end pedestals and 62 tie-down winches with chains and bridge plates for transporting trailer tractors saddleback style) | 1/75-1/80 | |
| Trailer Train Co. | eliminated 1/92 | |
| TTX Co. | added 1/92 | |
| TTX Co. (flat cars equipped with 32 tie-down winches with chains, may be equipped with pedestals, for transporting trailer tractors, large trucks and military vehicles) | Jan-94 | |
| TTX Co. (flat cars equipped with 32 tie-down winches with chains, may be equipped with pedestals, for transporting trailer tractors, large trucks and military vehicles, equipped with end of car cush.) | Jul-00 | |
| ITX | International Agricultural Corp. | Apr-38 |
| ITXX | Tiaxcelteca de Industrias, S.A. de C.V. | Jul-00 |
| IU | Illinois Union Ry. Co. | Apr-38 |
| Indianapolis Union Ry. | 1/52-1/94; eliminated 1/98 | |
| Indianapolis Union | Jul-00 | |
| IUX | Industrias Unidas de Nuevo Laredo, S.A. | 1/52-7/65; eliminated 10/65 |
| I&V | Iberia & Vermillion R.R. | Apr-38 |
| IWC | Inland Waterways Corp. | Jan-52 |
| IWRR | Intermountain Western R.R. | Jan-94 |
| IWSX | Illinois-Wisconsin Sand & Gravel Co. | 1/75-1/80; eliminated 10/80 |