| QAP | Quanah, Acme & Pacific Ry. Co. | 4/38-1/80; eliminated 7/81 |
| QBT | Quincy Bay Terminal Co. | added 10/92; 1/94-7/2000 |
| QC | Quebec Central Ry. | 4/38-10/57 |
| Quebec Central Ry. Co. | 1/75-1/94; eliminated 7/94 | |
| CP Rail System (Canadian Pacific Ltd.) | added 7/94; eliminated 1/97 | |
| Canadian Pacific Ry. Co. | added 1/97; 7/2000 | |
| QCCX | Trancisco Leasing Co. | added 10/96; eliminated 1/97 |
| Quantum Chemical Corp. | added 1/97 | |
| Quantum Chemical Corp., USI Division | eliminated 4/98 | |
| Equistar Chemicals, LP | added 4/98; 7/2000 | |
| QFFX | Quimica Fluor, S.A. de C.V. | added 4/93; 1/94-7/2000 |
| QGRY | Quebec Gatineau Ry., Inc. | added 10/98; 7/2000 |
| QLMX | Qilico Ltd. c/o SC Capital Corp. | added 10/94; eliminated 10/97 |
| QLP | Quebec Ry. Light & Power Co. | 4/38-4/70 |
| QLSX | Quaker City Live Stock Express (General American Transportation Corp.) | Apr-38 |
| QMRX | Merchants Despatch Transportation Co. | 10/57-7/65; eliminated 7/66 |
| QMS | Quebec, Montreal & Southern Ry. | 4/38-1/52 |
| QOCX | North American Car Co. | added 4/71; 1/75-1/80 |
| Quaker Oats Co. | 1/94-7/2000 | |
| QOHX | Quaker Oats Co. | added 10/80; 1/94-7/2000 |
| QOPX | General Electric Railcar Services Corp. | added 1/88; 1/94; eliminated 1/2000 |
| General Electric Rail Services Corp. | added 1/2000; 7/2000 | |
| QOTX | Quaker Oats Co.-Chemicals Division | added 10/80; eliminated 10/84 |
| QO Chemicals, Inc. | added 10/84; 1/94; eliminated 10/95 | |
| Great Lakes Chemical Corp., Specialty Chemicals | added 10/95; 7/2000 | |
| QPCX | The Quaker Petroleum Co. | Apr-38 |
| General Electric Railcar Services Corp. | Jan-94 | |
| General Electric Rail Services Corp. | Jul-00 | |
| QRC | Quebec Ry. Corp. Inc. | Jul-00 |
| QRR | Quincy R.R. Co. | 1/75-7/2000 |
| QRSX | Quality Railway Services & Supply Co. | 7/2000; eliminated 7/2001 |
| QSMX | Quaker State Oil Refining Corp. | 4/38-1/80; eliminated 10/87 |
| Quaker State Corp. | added 10/87; 1/94; eliminated 10/94; 7/2000 | |
| QSR | Quebec Southern Ry. | added 4/97; 7/2000; eliminated 4/2001 |
| Bangor and Aroostook Rd. Co. | added 4/2001 | |
| QTTX | Trailer Train Co. | Jul-65 |
| Trailer Train Co. (FMS flat cars specially equipped to provide for loading of agricultural equipment) | 10/66; eliminated 10/67 | |
| Trailer Train Co. | Apr-70 | |
| Trailer Train Co. (no definition) | 1/75-1/80 | |
| Trailer Train Co. | added 7/91; eliminated 1/92 | |
| TTX Co. | added 1/92 | |
| TTX Co. (heavy capacity special type flats AAR mechanical designations FD and FM ranging in capacity from 110-ton to 245-ton of various lengths) | Jan-94 | |
| TTX Co. (heavy capacity special type flats AAR mechanical designations FD and FM ranging in capacity from 105-ton to 370-ton of various lengths, equipped with end of car cush.) | Jul-00 | |
| QTX | Quaker City Tank Line (General American Transportation Corp.) | Apr-38 |
| QUAX | Quality Rail Services, LLC | added 1/2000; 7/2000; eliminated 4/2001 |