U
- UBTX - Union Camp Corporation
- UCCX - Union Carbide Canada, Ltd.
- UCEX - Union Electric Company
- UCFX - Union Carbide Corporation
- UCLX - Vulcan Minerals Company
- UCOX - Union Carbide Corporation
- UCPX - Essem Corporation
- UCR - Utah Coal Route
- UCRY - Utah Central Railway
- UELX - United Equipment Leasing Associates; Archer Daniels Midland
- UECZ - Union Elevator Co.
- UEEZ - Union Equity Elevator Co.
- UEFZ - Union Equity
- UFIX - Utility Fuels, Inc.
- UIWX - Riley Stoker Corporation
- UMP - Upper Merion and Plymouth Railroad
- UMPX - Upper Merion and Plymouth Leasing Company
- UNI - Unity Railways
- UNCX - Union Carbide Canada, Ltd.
- UNPX - Procor
- UNSX - Unitrain Services
- UO - Union Railroad (Oregon)
- UOCX - General American Transportation Corporation
- UP - Union Pacific Railroad
- UPAX - United Power Association
- UPB - Union Pacific Railroad
- UPFE - Pacific Fruit Express; Union Pacific Railroad
- UPM - Union Pacific Railroad (Milwaukee Road directed operations)
- UPOZ - United Parcel Service
- UPP - Union Pacific Railroad (passenger cars)
- UPRQ - Union Pacific Railroad End Of Train Devices
- UPRX - Union Pacific Fruit Express Company
- UPS - Union Pacific Railroad (slugs)
- UPY - Union Pacific Railroad (yard switchers)
- UTBX - Union Tank Car Company
- UTCX - Union Tank Car Company
- UTLX - Union Tank Car Company
- UTPX - Union Texas Petroleum Corporation
- UTR - Union Transportation
- UTTX - Trailer Train Company