Note: Marks ending in U are for container owners; marks ending in X are not common carriers; marks ending in Z are for trailers without flanged wheels. All other marks are of common-carrier railroads.

• QC - Quebec Central Railway
• QCCX - Quantum Chemical Corporation
• QGRY - Quebec Gatineau Railway
• QNSX - Rail Enterprises Inc. (Doing Business As: Quebec North Shore and Labrador Railway on behalf of Iron Ore Company of Canada)
• QOCX - GE Rail Services
• QOHX - Quaker Oats Company
• QOTX - Quaker Oats Company (Chemicals Division)
• QRR - Quincy Railroad
• QSMX - Quaker State Oil Refining Corporation
• QSOX - Quaker State Oil Refining Corporation
• QTTX - TTX Corporation