An overhead transport (OHT) system with multiple levels of rails for the transport of semiconductor workpieces is provided. A first vehicle is configured to travel on, and move a semiconductor workpiece along, a first rail. A second vehicle is configured to travel on, and move the semiconductor workpiece along, a second rail overlying the first rail. A controller is configured to control the first and second vehicles to transfer the semiconductor workpiece along the first and second rails, between process or inspection tools. A method for transferring semiconductor workpieces across multiple levels of rails is also provided.