| 301 | HTTX | TTX Co. | added 1/1992; 1/1994-7/2005 | | NAKINA |
| 302 | HTTX | Trailer Train Co. | added 4/1968; 10/1968-1/1980; eliminated 1/1992 | | NAKINA |
| 303 | HTTX | Trailer Train Co. | added 4/1968 | | NAKINA |
| 304 | HTTX | Trailer Train Co. (FMS flats specifically equipped with heavy duty chains, snubbers and turnbuckles, secured to moveable and retractable tiedown anchors, in four longitudinal channels) | 10/1968 | | NAKINA |
| 305 | HTTX | Trailer Train Co./TTX Co. (flat cars equipped with 38 heavy duty chains, with snubbers and turnbuckles, each secured to movable and retractable tie-down winches in 4 longitudinal channels, for transporting large earth moving equipment) | 4/1970-1/1994 | | NAKINA |
| 306 | HTTX | TTX Co. (flat cars equipped with 38 heavy duty chains, with snubbers and turnbuckles, each secured to movable and retractable tie-down winches in 4 longitudinal channels, for transporting large earth-moving equipment, equipped with end of car cush.) | 7/2000 | | NAKINA |
| 307 | HTTX | Trailer Train Co. | added 4/68 | | NRHS |
| 308 | HTTX | Trailer Train Co. (FMS flats specifically equipped with heavy duty chains, snubbers and turnbuckles, secured to moveable and retractable tiedown anchors, in four longitudinal channels) | Oct-68 | | NRHS |
| 309 | HTTX | Trailer Train Co. | Apr-70 | | NRHS |
| 310 | HTTX | Trailer Train Co. (flat cars equipped with 38 heavy duty chains, with snubbers and turnbuckles, each secured to movable and retractable tie-down winches in 4 longitudinal channels, for transporting large earth moving equipment) | 1/75-1/80; eliminated 1/92 | | NRHS |
| 311 | HTTX | TTX Co. (flat cars equipped with 38 heavy duty chains, with snubbers and turnbuckles, each secured to movable and retractable tie-down winches in 4 longitudinal channels, for transporting large earth moving equipment) | added 1/92; 1/94 | | NRHS |
| 312 | HTTX | TTX Co. (flat cars equipped with 38 heavy duty chains, with snubbers and turnbuckles, each secured to movable and retractable tie-down winches in 4 longitudinal channels, for transporting large earth-moving equipment, equipped with end of car cush.) | Jul-00 | | NRHS |
| 313 | HTTX | TTX COMPANY | | | PWRC |
| 314 | HTTX | Trailer Train (TTX Corporation) | | | RSRV |
| 315 | HTTX | TTX COMPANY | | | SCAC4 |
| 316 | HTU | Yunnan Hong Tu Airlines Co., Ltd. | IATA: A6, Callsign: Hong Land, IATA Prefix: 389, Name changed to Air Travel Co., Ltd. ICAO Designator changed to OTC. | PRC | AVCODES |
| 317 | HTUU | KIECA TRANS | | PL | BIC |
| 318 | HTVN | HETCO VAN LINES INC | | | SCAC4 |
| 319 | HTW | Hoosac Tunnel & Wilmington | 1/1994-7/2000 | | NAKINA |
| 320 | HTW | Hoosac Tunnel & Wilmington | 1/94-7/2000 | | NRHS |
| 321 | HTW | *HOOSAC TUNNEL AND WILMINGTON RAILROAD CO. | | | PWRC |
| 322 | HTW | Hoosac Tunnel and Wilmington | | | RSRV |
| 323 | HTWA | HODGE TRANSPORTATION & WAREHOUSING | | | SCAC4 |
| 324 | HTWC | HALAL TOWING INC | | | SCAC4 |
| 325 | HTWD | HOOSIER TRADEWINDS INC | | | SCAC4 |
| 326 | HTWF | H2O FARMS | | | SCAC4 |
| 327 | HTWG | HARTWIG, C TRANSPORT | | | SCAC4 |
| 328 | HTWH | HAMILTON, E H TRUCKING & | | | SCAC4 |
| 329 | HTWI | HARTWIG AIRCRAFT FUEL CELL REPAIR I | | | SCAC4 |
| 330 | HTWJ | HTW LOGISTICS | | | SCAC4 |
| 331 | HTWK | HAULMARK TRUCKING INC | | | SCAC4 |
| 332 | HTWM | HATCHER, WILLIAM RAY TRUCKING CO | | | SCAC4 |
| 333 | HTWQ | HAMNER TOWING INC | | | SCAC4 |
| 334 | HTWT | HOT WATER TRUCKING LLC | | | SCAC4 |
| 335 | HTWV | H TWO (2) | | | SCAC4 |
| 336 | HTWY | HUTTER, WALLY OIL CO | | | SCAC4 |
| 337 | HTX | Houston Tank Car Co. | 1/1952-10/1957; eliminated 4/1960 | | NAKINA |
| 338 | HTX | The Hewitt Transportation Co. (The Hewitt Soap Co., Inc.) | 7/1937-4/1938 | | NAKINA |
| 339 | HTX | The Hewitt Soap Co., Inc. | 7/1935 | | NAKINA |
| 340 | HTX | The Hewitt Soap Co., Inc. | Apr-38 | | NRHS |
| 341 | HTX | Houston Tank Car Co. | 1/52-10/57 | | NRHS |
| 342 | HTX | HEWETT SOAP CO., INC. (HEWITT TRANSPORTATION CO.) | | | PWRC |
| 343 | HTXC | HEAT TRANSFER LLC | | | SCAC4 |
| 344 | HTXG | HIRTE TRANSFER & STORAGE | | | SCAC4 |
| 345 | HTXI | HOPE TRANSPORT INC | | | SCAC4 |
| 346 | HTXL | HIALEAH TRANSPORT LLC | | | SCAC4 |
| 347 | HTXN | HEARNE TRANSPORTATION | | | SCAC4 |
| 348 | HTXO | HAT TRANSPORT LLC | | | SCAC4 |
| 349 | HTXT | HOLBORN TRANSPORT | | | SCAC4 |
| 350 | HTXU | JINGMEN HONGTU SPECIAL AIRCRAFT MANUFACTURING CO LTD | | PRC | BIC |
| 351 | HTY | Global Aeronautics Solutions Corp, S.L. t/a HELITY Copter Airlines | Callsign: Helity, | E | AVCODES |
| 352 | HTYI | HARTLEY TRANSPORTATION INC | | | SCAC4 |
| 353 | HTZI | HEITZ TRUCKING, INC | | | SCAC1 |
| 354 | HTZI | HEITZ TRUCKING INC | | | SCAC4 |
| 355 | HTZL | HETZELS OVERLAND TRANSPORT INC | | | SCAC4 |
| 356 | HTZS | HOTZ ENVIRONMENTAL SERVICES INC | | | SCAC4 |
