| 1001 | UMP | Upper Merion and Plymouth R.R. Co. | 7/1954-7/2005 | | NAKINA |
| 1002 | UMP | Upper Merion and Plymouth R.R. Co. | 10/57-7/2000 | | NRHS |
| 1003 | UMP | UPPER MERION & PLYMOUTH RAILROAD COMPANY | | | PWRC |
| 1004 | UMP | Upper Merion and Plymouth Railroad | | | RSRV |
| 1005 | UMP | Upper Merion and Plymouth R.R. Co. | | USA | TRAFFIC |
| 1006 | UMPI | UNIVERSAL METAL PRODUCTS INC | | | SCAC4 |
| 1007 | UMPX | Upper Merion & Plymouth Leasing Co. | added 4/1980; eliminated 4/1989 | | NAKINA |
| 1008 | UMPX | Upper Merion and Plymouth Leasing Co. | added 4/1989; 1/1994 | | NAKINA |
| 1009 | UMPX | Upper Merion and Plymouth Co. | eliminated 4/1996 | | NAKINA |
| 1010 | UMPX | First Union Rail | added 4/1996; 7/2000-7/2005 | | NAKINA |
| 1011 | UMPX | Upper Merion & Plymouth Leasing Co. | added 4/80; eliminated 4/89 | | NRHS |
| 1012 | UMPX | Upper Merion and Plymouth Leasing Co. | added 4/89; 1/94 | | NRHS |
| 1013 | UMPX | Upper Merion and Plymouth Co. | eliminated 4/96 | | NRHS |
| 1014 | UMPX | First Union Rail | added 4/96; 7/2000 | | NRHS |
| 1015 | UMPX | FIRST UNION RAIL [UPPER MERION & PLYMOUTH LEASING COMPANY] | | | PWRC |
| 1016 | UMPX | Upper Merion and Plymouth Leasing | | | RSRV |
| 1017 | UMPX | FIRST UNION RAIL | | | SCAC4 |
| 1018 | UMRI | USA MEX REFRIGERATED TRANSPORT INC | | | SCAC4 |
| 1019 | UMS | Unitemp-M, Limited Liability Industrial Company | Callsign: Topaz, | RUS | AVCODES |
| 1020 | UMS | Unitemp-M | | | ICAO1 |
| 1021 | UMSA | UGLAND MARINE SERVICES AS | | | SCAC4 |
| 1022 | UMSC | UNIVERSITY MOVING & STORAGE COMPANY | | | SCAC4 |
| 1023 | UMSD | UNIVERSITY MOVING & STORAGE COMPANY | | | SCAC4 |
| 1024 | UMST | UMS TRUCKING | | | SCAC4 |
| 1025 | UMSX | UNITED STEELWORKERS OF AMERICA | | | PWRC |
| 1026 | UMSZ | INTERMODAL SERVICES, INC. | | | PWRC |
| 1027 | UMSZ | Maersk Equipment Services | | | RSRV |
| 1028 | UMSZ | MAERSK EQUIPMENT SERVICE COMPANY | | | SCAC4 |
| 1029 | UMTC | ULTRA MARINE TRANSPORT CORP | | | SCAC4 |
| 1030 | UMTI | U MOTORS INC | | | SCAC4 |
| 1031 | UMTU | ULTRAMAR LLC | | RUS | BIC |
| 1032 | UMVS | UNIVERSITY MOVING & STORAGE CO | | | SCAC4 |
| 1033 | UMXF | UMEX FREIGHT SYSTEM INC | | | SCAC4 |
| 1034 | UMXU | CSX TRANSPORTATION INC | | | SCAC4 |
| 1035 | UN | Transaero Airlines | ICAO: TSO, Callsign: Transoviet, IATA Prefix: 670, Founded on 28 December 1990. Commenced operations on 05 November 1991. Ceased operations on 25 October 2015. | RUS | AVCODES |
| 1036 | UN | Universair, SA | ICAO: UNA, Callsign: Universair, Commenced operations in 1986. Merged into Meridiana Air, SA on 01 March 1991. | E | AVCODES |
| 1037 | UN | East Coast Airlines Limited | | AUS | AVCODES |
| 1038 | UN | Business Aviation Asia Limited | | HK | AVCODES |
| 1039 | UN | Business Aviation Asia Limited | ICAO: UNA, | HK | AVCODES |
| 1040 | UNA | Universair, SA | IATA: UN, Callsign: Universair, Commenced operations in 1986. Merged into Meridiana Air, SA on 01 March 1991. | E | AVCODES |
| 1041 | UNA | Business Aviation Asia Limited | IATA: UN, | HK | AVCODES |
| 1042 | UNAA | UNIQUE FREIGHT | | | SCAC4 |
| 1043 | UNAC | UNITED FUEL SERVICE | | | SCAC4 |
| 1044 | UNACOP | Unacop d.o.o. | City: Beograd | SR | TRAFFIC |
| 1045 | UNAP | UNITED EXPRESS LTD | | | SCAC4 |
| 1046 | UNAR | UNION AIR TRANSPORT GMBH | | | SCAC4 |
| 1047 | UNAS | UNION AGRICOLA S A DE C V | | | SCAC4 |
| 1048 | UNAU | UNITEAM AUSTRALASIA PTY LTD | | | SCAC4 |
| 1049 | UNBC | UNITED BULB COMPANY | | | SCAC4 |
| 1050 | UNBD | UNIONVILLEDEPOT.COM | | | SCAC4 |
| 1051 | UNBL | UNITED BLOWER INC | | | SCAC4 |
| 1052 | UNBX | Union Boiler Repair, Fabrication & Erection Co., Inc. | added 7/1970; eliminated 10/1972 | | NAKINA |
| 1053 | UNBX | Union Boiler Repair | added 10/1972; eliminated 11/1973 | | NAKINA |
| 1054 | UNBX | General Electric Railcar Services Corp. | added 4/1999; eliminated 1/2000 | | NAKINA |
| 1055 | UNBX | General Electric Rail Services Corp. | added 1/2000; 7/2000-7/2005 | | NAKINA |
| 1056 | UNBX | Union Boiler Repair, Fabrication & Erection Co., Inc. | added 7/70; eliminated 10/72 | | NRHS |
| 1057 | UNBX | Union Boiler Repair | added 10/72; eliminated 11/73 | | NRHS |
| 1058 | UNBX | General Electric Railcar Services Corp. | added 4/99; eliminated 1/2000 | | NRHS |
| 1059 | UNBX | General Electric Rail Services Corp. | added 1/2000; 7/2000 | | NRHS |
| 1060 | UNBX | *UNION BOILER REPAIR | | | PWRC |
| 1061 | UNBX | +GENERAL ELECTRIC RAIL SERVICES CORP. | | | PWRC |
| 1062 | UNBX | General Electric Rail Services Corporation | | | RSRV |
| 1063 | UNBX | GENERAL ELECTRIC RAILCAR SERVICES C | | | SCAC4 |
| 1064 | UNC | Uni-fly A/S | Callsign: Unicopter, | DK | AVCODES |
| 1065 | UNC | Uni-Fly | | | ICAO1 |
| 1066 | UNCA | UNKNOWN CANADIAN AIR CARRIER | | | SCAC1 |
| 1067 | UNCA | UNITED CAB ASSOCIATION | | | SCAC4 |
| 1068 | UNCB | UNITED TOTE COMPANY | | | SCAC4 |
| 1069 | UNCC | UNICOLD CORPORATION | | | SCAC4 |
| 1070 | UNCD | UNICEL LIMITED | | | SCAC4 |
| 1071 | UNCF | UNISEA SHIPPING CV | | | SCAC4 |
| 1072 | UNCG | UNION CARTAGE CO | | | SCAC1 |
| 1073 | UNCH | UNICHAIRS INC | | | SCAC4 |
| 1074 | UNCL | UNITED CONTAINERLINES INC | | | SCAC4 |
| 1075 | UNCR | UNITED CRANE & RIGGING | | | SCAC4 |
| 1076 | UNCU | UNI-LOGISTICS SP. Z O.O. | | PL | BIC |
| 1077 | UNCU | UNIVERSAL CONTAINER LTD | | | SCAC4 |
| 1078 | UNCX | United Central Oil Corp. (Crown Central Petroleum Corp.) | 7/1930 | | NAKINA |
| 1079 | UNCX | Universal Car Co. | 7/1935-4/1938 | | NAKINA |
| 1080 | UNCX | Union Carbide Canada Ltd., Plastics & Chemicals | added 7/1981; eliminated 4/1987 | | NAKINA |
| 1081 | UNCX | General Electric Railcar Services Corp. | eliminated 4/1992 | | NAKINA |
| 1082 | UNCX | CGTX, Inc. | added 4/1992; 1/1994-7/2000; eliminated 4/2002 | | NAKINA |
| 1083 | UNCX | GATX Rail Canada Corp. | added 4/2002; 7/2005 | | NAKINA |
| 1084 | UNCX | Universal Car Co. | Apr-38 | | NRHS |
| 1085 | UNCX | Union Carbide Canada Ltd., Plastics & Chemicals | added 7/81; eliminated 4/87 | | NRHS |
| 1086 | UNCX | General Electric Railcar Services Corp. | eliminated 4/92 | | NRHS |
| 1087 | UNCX | CGTX, Inc. | added 4/92; 1/94-7/2000 | | NRHS |
| 1088 | UNCX | *CROWN CENTRAL PETROLEUM CO. [UNITED CENTRAL OIL CORP.] | | | PWRC |
| 1089 | UNCX | *UNIVERSAL CAR CO. | | | PWRC |
| 1090 | UNCX | *UNION CARBIDE CANADE, LTD. | | | PWRC |
| 1091 | UNCX | +GATX RAIL CANADA CORP. [(CGTX)GENERAL ELECTRIC RAILCAR SERVICES CORP.] | | | PWRC |
| 1092 | UNCX | Union Carbide Canada | | | RSRV |
| 1093 | UNCX | GATX RAIL CANADA CORPORATION | | | SCAC4 |
| 1094 | UNCZ | UNIVERSAL COOPERATIVE | | | PWRC |
| 1095 | UND | Atuneros Unidos de California, S.A. de C.V. | Callsign: Atuneros Unidos, | MEX | AVCODES |
| 1096 | UND | Atuneros Unidos de California | | | ICAO1 |
| 1097 | UNDB | UNITED OCEAN LINES | | | SCAC4 |
| 1098 | UNDG | UNITED LOGISTIC INC | | | SCAC4 |
| 1099 | UNDH | UNDERWOOD, JEFF INC | | | SCAC4 |
| 1100 | UNDI | UNITED OIL | | | SCAC4 |
