SDN and virtualization promise to simplify transport network control, while adding management flexibility and allowing the rapid development of new services by enabling programmatic control of transport networks and equipment. A consensus transport SDN (T-SDN) architecture using standards- based system inter-component interfaces further enables flexible composition of functionally unified systems from among vendor-diverse software and hardware components. In this article we discuss the high-level reference T-SDN architecture emerging from early experience, standards, and related industry activities, with attention paid to critical standards-based system interface methodologies. We examine key T-SDN use cases including service bandwidth on demand, virtual transport network services, and multi-layer control convergence and resource optimization.