| UACX | Universal Atlas Cement Co. | Jan-52 |
| Universal Atlas Cement Division of U.S. Steel Corp. | eliminated 7/65 | |
| UAIX | United Asphalts, Inc. | added 10/92; 1/94-7/2000 |
| UAWX | Union Acid Works, Inc. (Standard Wholesale Phosphate & Acid Works, Inc.) | Apr-38 |
| UBTX | Union Bag & Paper Corp. | added 7/55 |
| Union Bag-Camp Paper Corp. | 10/57-7/65; eliminated 7/66 | |
| Union Camp Corp. | added 7/66; 10/66-1/80; eliminated 10/87 | |
| UCBX | United Carbon Co., Inc. | 4/38-10/66; eliminated 1/69 |
| Ashland Chemical Co., Division of Ashland Oil & Refining Co., Carbon Black & Synthetic Rubber Division | added 1/69; 4/70; eliminated 4/71 | |
| Ashland Chemical Co., Division of Ashland Oil Inc., Carbon Black & Synthetic Rubber Division | added 4/71 | |
| Ashland Chemical Co., Division of Ashland Oil Inc. | Jan-75 | |
| UCCX | United Contracting Co. | 1/52-4/61 |
| Union Carbide Canada Ltd. | added 10/69; 4/70-1/80; eliminated 1/85; 1/94-7/2000 | |
| UCEX | Union Electric Co. | added 10/80; eliminated 10/84; 1/94; added 1/97; eliminated 4/97; 7/2000 |
| UCFX | Union Carbide Corp., Chemicals and Plastics Operations Division | added 4/71; 1/75; eliminated 1/79 |
| Union Carbide Corp., Chemicals and Plastics Division | added 1/79; 1/80; eliminated 7/80 | |
| Union Carbide Corp. | added 7/80; eliminated 10/80 | |
| Union Carbide Corp., Chemicals and Plastics Division | added 10/80; eliminated 5/83 | |
| Union Carbide Corp. | added 5/83; eliminated 1/93 | |
| Union Carbide Chemicals and Plastics Co., Inc. | added 1/93; 1/94; eliminated 1/95; eliminated 4/95 | |
| Union Carbide Corp. | added 1/95; added 4/95; 7/2000 | |
| UCIR | Union County Industrial R.R. Co. | added 7/96; 7/2000 |
| UCIX | Unichema Chemicals, Inc. | added 1/86; 1/94; eliminated 4/96 |
| UCLX | Union Chemical Carline, Inc. | added 7/56; 10/57-4/70 |
| Vulcan Materials Co., Chemicals Division | 1/75-7/2000 | |
| UCMX | United Commercial Co., Inc. | Apr-38 |
| United Clays, Inc. | added 7/90; 1/94-7/2000 | |
| UCOX | Union Carbide Corp., Chemicals Division | added 7/64; 7/65-10/66; eliminated 10/67 |
| Union Carbide Corp., Chemicals and Plastics | added 10/67; eliminated 1/68 | |
| Union Carbide Corp., Chemicals and Plastics Operations Division | added 1/68; 4/70-1/75; eliminated 1/79 | |
| Union Carbide Corp., Chemicals and Plastics Division | added 1/79; 1/80; eliminated 7/80 | |
| Union Carbide Corp. | added 7/80; eliminated 10/80 | |
| Union Carbide Corp., Chemicals and Plastics Division | added 10/80; eliminated 5/83 | |
| Union Carbide Corp. | added 5/83 | |
| Union Carbide Corp., Chemicals and Plastics Co., Inc. | eliminated 1/93 | |
| Praxair, Inc. | added 1/93; 1/94; eliminated 4/96; 7/2000 | |
| UCPX | Union Carbide Corp., Chemicals Division | 7/65-10/66; eliminated 10/67 |
| Union Carbide Corp., Chemicals and Plastics | added 10/67; eliminated 1/68 | |
| Union Carbide Corp., Chemicals and Plastics Operations Division | added 1/68; eliminated 4/69 | |
| Essem Corp. | added 4/69; 4/70-1/80 | |
| UCR | Utah Coal Route | 4/38-1/80 |
| UCRY | Utah Central Ry. Co. | added 1/96; eliminated 4/97; 7/2000 |
| UCSX | Union Camp Corp. | added 10/88; 1/94-7/2000 |
| UCX | Uvalde Construction Co. | 1/52-4/61 |
| UDAX | North American Car Corp. | Jan-52 |
| UDIX | Utilities Distributors, Inc. | Jan-52 |
| UDX | United Dairy Co. | Apr-38 |
| UE | Union Electric Ry. | Jan-52 |
| UELX | United Equipment Leasing Associates | added 7/77; 1/80; eliminated 4/88 |
| ADM Transportation Co. | added 4/88; 1/94-7/2000 | |
| UFGX | United Fuel Gas Co. | Jan-52 |
| UFIX | Utility Fuels, Inc. | added 7/78; 1/80-1/94; eliminated 4/94 |
| Houston Lighting & Power Co. | added 4/94; eliminated 7/2000 | |
| Reliant Energy HL&P | added 7/2000; 7/2000 | |
| UGPX | United Gas Pipe Line Co. | Apr-38 |
| UHRX | Upper Hudson River R.R. Corp. | Jul-00 |
| UIC | Utah Idaho Central R.R. | 4/38-1/52 |
| UIWX | Union Iron Works, subsidiary of Riley Stoker Corp. | added 1/68; eliminated 4/70 |
| Riley Stoker Corp. | added 4/70; 4/70-1/94; eliminated 7/98 | |
| UKKX | United Carbon Co. | Apr-38 |
| ULCX | Unit Load Car Corp. | added 7/59; 4/61-4/70; eliminated 10/74 |
| UMP | Upper Merion and Plymouth R.R. Co. | 10/57-7/2000 |
| UMPX | Upper Merion & Plymouth Leasing Co. | added 4/80; eliminated 4/89 |
| Upper Merion and Plymouth Leasing Co. | added 4/89; 1/94 | |
| Upper Merion and Plymouth Co. | eliminated 4/96 | |
| First Union Rail | added 4/96; 7/2000 | |
| UNBX | Union Boiler Repair, Fabrication & Erection Co., Inc. | added 7/70; eliminated 10/72 |
| Union Boiler Repair | added 10/72; eliminated 11/73 | |
| General Electric Railcar Services Corp. | added 4/99; eliminated 1/2000 | |
| General Electric Rail Services Corp. | added 1/2000; 7/2000 | |
| UNCX | Universal Car Co. | Apr-38 |
| Union Carbide Canada Ltd., Plastics & Chemicals | added 7/81; eliminated 4/87 | |
| General Electric Railcar Services Corp. | eliminated 4/92 | |
| CGTX, Inc. | added 4/92; 1/94-7/2000 | |
| UNI | Unity Railways Co. | 1/75-1/80; eliminated 7/86 |
| UNIX | Unigas, Inc. | 7/65-4/70 |
| UNPX | Union Petroleum Co. (Sinclair Refining Co.) | Apr-38 |
| Sinclair Refining Co. | 1/52; eliminated 1/58 | |
| Union Tank Car Co. | 10/57; added 1/58; 4/61; added 7/64; 7/65-10/66; added 1/70; 4/70; eliminated 1/74 | |
| Procor Ltd. | added 1/74; 1/75-7/2000 | |
| UNSX | Unitrain Services (agent for Iowa Power and Light Co.) | added 10/80; eliminated 4/93 |
| Midwest Power Systems | added 4/93; 1/94; eliminated 4/96 | |
| MidAmerican Energy Co. | added 4/96; 7/2000 | |
| UNTX | Unitrain, Inc. | added 10/96; eliminated 4/98 |
| MRC Rail Services, LLC | added 4/98; 7/2000 | |
| UNVX | Universal Corp. | added 10/60; 4/61 |
| UO | Union R.R. of Oregon | 1/75; added 7/79; 1/80; eliminated 4/86; eliminated 4/89 |
| UOCX | Union Oil Co. of California | 4/38-1/52 |
| General American Transportation Corp. | 10/57-7/65 | |
| Union Oil Co. of California (General American Transportation Corp.) | Oct-66 | |
| General American Transportation Corp. | 4/70-1/94; eliminated 1/98 | |
| General American Marks Co. | added 1/98; 7/2000 | |
| UOEX | Union Oil Co. of California | added 10/84; eliminated 1/87 |
| UNOCAL | added 1/87; eliminated 7/92 | |
| UNO-VEN | added 7/92; 1/94; eliminated 4/98; 7/2000 | |
| UOKX | Union Oil Co. of Canada Ltd. (Union Oil Co. of California) | Apr-38 |
| UOPX | UOP, A New York Partnership | added 1/97; eliminated 7/97; 7/2000 |
| UP | Union Pacific R.R. Co. | 4/61-7/2000 |
| UPAX | United Power Association | added 4/82; 1/94; eliminated 4/2000 |
| Great River Energy | added 4/2000; 7/2000 | |
| UPCX | Universal Portland Cement Co. (Universal Atlas Cement Co.) | Apr-38 |
| Union Petroleum Corp. | added 1/62; 7/65-4/70 | |
| Home Petroleum Corp. | 1/75-1/80 | |
| USPCI c/o Laidlaw Environmental Services, Inc. | added 10/95; eliminated 4/96 | |
| Laidlaw Environmental Services, Inc. | added 4/96; 7/2000 | |
| UPFE | Union Pacific R.R. Co. | 1/75-7/2000 |
| UPFX | United States Pipe and Foundry Co. | Apr-38 |
| UPM | Union Pacific R.R. Co. | Jan-80 |
| UPP | Union Pacific R.R. Co. (passenger cars) | 1/94-7/2000 |
| UPR | Union Pacific R.R. Co. | 1/94-7/2000 |
| UPRX | Ultra-Penn Refining Co. | Apr-38 |
| Union Pacific Fruit Express Co. | added 1/78; 1/80; eliminated 7/84 | |
| UPX | Unity Petroleum Corp. | Apr-38 |
| UPY | Union Pacific R.R. | Jul-00 |
| URC | United Railways Co. | Apr-38 |
| URCX | United Refining Co. | 4/38-4/70 |
| URDX | United States Steel Corp., Resource Development Group | added 10/80; eliminated 4/85 |
| Arch of Kentucky, Inc. | added 4/85; 1/94; eliminated 4/94; 7/2000 | |
| URGX | UniCapital Rail Group | Jul-00 |
| URMX | Union Refrigerator Transit Lines, a Division of the General American Transportation Corp. | 7/65-10/66 |
| Union Refrigerator Transit Lines | eliminated 7/67 | |
| URR | Union R.R. Co. (Pittsburgh, PA) | 1/75-7/2000 |
| URTX | Union Refrigerator Transit Lines, a Division of the General American Transportation Corp. | 4/38-4/70; eliminated 10/71 |
| General American Transportation Corp., Freight Car Division | added 10/71 | |
| General American Transportation Corp. | 1/75-1/80; eliminated 10/92 | |
| URY | Union Ry. of Memphis | 1/75-7/2000 |
| USAX | Dept. of the Army, Office of the Chief of Transportation, Movements Division, Tank Car Section | 1/52-10/57 |
| Dept. of the Army, Transportation Supply and Maintenance Command | eliminated 7/58 | |
| Dept. of the Army, U.S. Army Transportation Supply and Maintenance Command | added 7/58 | |
| Dept. of the Army, Office of the Chief of Transportation, Movements Division, Tank Car Section | 1/52-4/61 | |
| Dept. of Defense, Military Traffic Management Agency | eliminated 4/62 | |
| Dept. of Defense, Defense Traffic Management Service | added 4/62; eliminated 7/65 | |
| Dept. of Defense, Military Traffic Management and Terminal Service | added 7/65; 7/65-4/70 | |
| Dept. of Defense, Military Traffic Management Command | 1/75; eliminated 7/75 | |
| Dept. of Defense, Troop Support and Aviation Material Readiness Command (intra-plant service) | 1/80-7/2000 | |
| USBX | U.S. Borax & Chemical Corp. | added 7/64; 7/65-10/66; eliminated 1/69 |
| North American Car Corp. | added 7/82 | |
| General Electric Railcar Services Corp. | 1/94; eliminated 4/99 | |
| The CIT Group/Capital Finance, Inc. | added 4/99; 7/2000 | |
| USCX | United States Tank Car Corp. | added 10/72; eliminated 1/75 |
| William G. Simon, Trustee | added 1/75; 1/75; eliminated 7/75 | |
| William G. Simon | added 7/75; 1/80-7/2000 | |
| USEX | United States Railway Equipment Co. | added 4/55; 10/57-1/75; eliminated 7/79 |
| United States Leasing Co. | added 7/79 | |
| United States Railway Leasing Co. | 1/80; eliminated 10/80 | |
| Evans Railcar Leasing Co. | added 10/80; eliminated 4/88 | |
| Itel Rail Corp. | added 4/88; 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 | |
| USFX | Strick Division, Fruehauf Corp. | added 4/61; 4/61-7/65; eliminated 7/66 |
| USGX | Union Sugar Co. | added 1/82; eliminated 1/87 |
| Union Sugar Division, Holly Sugar Corp. | added 1/87; added 7/90 | |
| Holly Sugar Corp. | added 7/90; 1/94; eliminated 4/94 | |
| USIX | U.S. Industrial Alcohol Co. | Apr-38 |
| U.S. Industrial Chemicals Co., Division of National Distillers and Chemical Corp. | 1/80; eliminated 7/87 | |
| USI Chemicals Co., Division of National Distillers & Chemical Corp. | added 7/87; eliminated 7/88 | |
| Quantum Chemical Corp., USI Division | added 7/88; 1/94; eliminated 4/98 | |
| Equistar Chemicals, LP | added 4/98; 7/2000 | |
| USLD | Detroit, Toledo and Ironton R.R. Co. | 4/70-1/80; eliminated 1/80 |
| USLF | St. Louis-San Francisco Ry. Co. | 4/70-1/80; eliminated 1/81 |
| Burlington Northern Inc. | added 1/81; eliminated 7/81 | |
| Burlington Northern R.R. Co. | added 7/81 | |
| USLX | United States Railway Equipment Co. | eliminated 10/67 |
| United States Railway Leasing Co. | added 10/67; 4/70; eliminated 1/73 | |
| United States Railway Equipment Co. | added 1/73; 1/75; eliminated 7/79 | |
| United States Leasing Co. | added 7/79 | |
| United States Railway Leasing Co. | 1/80; eliminated 10/80 | |
| Evans Railcar Leasing Co. | added 10/80; eliminated 4/88 | |
| Itel Rail Corp. | added 4/88; 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 | |
| USMX | U.S. Steel Mining Co., Inc. | Jul-00 |
| USNX | United States Navy | Apr-38 |
| Dept. of the Navy, Bureau of S. & A. | 1/52-10/57 | |
| U.S. Navy | Apr-61 | |
| Dept. of Defense, Military Traffic Management Agency | eliminated 4/62 | |
| Dept. of Defense, Defense Traffic Management Service | added 4/62; eliminated 7/65 | |
| Dept. of Defense, Military Traffic Management and Terminal Service | added 7/65; 7/65-4/70 | |
| Dept. of Defense, Military Traffic Management Command | 1/75; eliminated 7/75 | |
| Dept. of Defense, Military Traffic Management Command (intra-plant service) | 1/80-7/2000 | |
| USOX | Dept. of the Army, Office of the Chief of Transportation, Movements Division, Tank Car Section | Jan-52 |
| USPX | U.S. Phosphoric Products Corp. | Apr-38 |
| U.S. Pipe and Foundry Co. | added 1/60; 4/61-10/66; eliminated 4/70 | |
| U.S. Plastics Corp. | 1/94-7/2000 | |
| USQX | Quartermaster Corps, War Department, U.S.A. | Apr-38 |
| Dept. of the Army, Office of the Chief of Transportation, Movement Division, Tank Car Section | Jan-52 | |
| USRX | United States Rubber Products, Inc. | Apr-38 |
| United States Rubber Co. | 1/52-10/66; eliminated 7/67 | |
| Uniroyal, Inc. | added 7/67; eliminated 4/68 | |
| U.S. Railcar, Inc. | Jan-94 | |
| USSX | United States Sugar Corp. | 1/52-4/61; eliminated 1/65 |
| United States Steel Corp. | 1/80; eliminated 7/87 | |
| Aristech Chemical Corp. | added 7/87; eliminated 7/88 | |
| USTX | Union Starch & Refining Co. | 4/38-7/65; eliminated 7/66; eliminated 10/66 |
| General American Transportation Corp. | added 10/66; 10/66; eliminated 1/68; 4/70 | |
| USWX | USA Waste Services, Inc. | added 10/96; eliminated 4/97; 7/2000 |
| UT | Union Terminal Ry. Co. | 1/52-4/70 |
| Union Terminal Ry. Co. (of St. Joseph, MO) | Jan-75 | |
| Missouri Pacific System | eliminated 1/79 | |
| UTAH | Utah Ry. Co. | 10/57-7/2000 |
| UTAX | United Transportation, Inc. | 1/75-1/80 |
| UTBX | Union Tank Car Co. | 1/75; added 4/75; 1/80-1/94 |
| UTCO | Union Traction Co. | Apr-38 |
| UTCX | Union Tank Car Co. | 4/38-7/2000 |
| UTLX | Union Tank Car Co. | 4/38-7/65 |
| Union Tank Line Co. (Union Tank Car Co.) | Oct-66 | |
| Union Tank Car Co. | 4/70-7/2000 | |
| UTPX | Union Texas Petroleum Co. | Jan-80 |
| Union Texas Petroleum Corp. | added 4/80; eliminated 10/84 | |
| Union Texas Products Corp. | added 10/84; eliminated 10/86 | |
| Suburban Propane Gas Corp. | eliminated 7/88 | |
| Suburban Propane-Division of Quantum Chemical Corp. | added 7/88; 1/94; eliminated 4/97 | |
| Suburban Propane L.P. | added 4/97; 7/2000 | |
| UTR | Union Transportation | 1/75-1/80 |
| UTTX | Trailer Train Co. | added 1/92 |
| TTX Co. | added 1/92 | |
| TTX Co. (5 platform articulated TOFC cars with rigid hitches, capable of carrying 1 trailer per platform, 40-48' long and up to 102" wide, with nose-mounted refrigeration unit between lift on/lift off facilities only) | 1/94-7/2000 | |
| UTX | Union Tank Car Co. | 4/38-4/70; eliminated 7/73 |
| UUAX | Up-Up & Away, Inc. | 1/94; added 1/97; eliminated 4/97; 7/2000 |
| UV | Unadilla Valley Ry. | 4/38-4/70 |