The increasing deployment of distributed energy resources (DERs) is disrupting every aspect of power system operations, from retail distribution to wholesale production and transmission. This paper reports on the development of an agentbased test system enabling the study of new transactive energy system (TES) designs to ensure the reliable efficient operation of integrated transmission and distribution (ITD) systems with growing DER penetration. This ITD test system is used to explore the ability of a non-profit Distribution System Operator (DSO), participating within an ITD system, to use an innovative TES design to manage the power usage of DER devices in accordance with the local goals and constraints of DER owners, and to extract flexible ancillary services from DER devices in return for appropriate market-based compensation.