The Metrolinx Economic Analysis team commissioned Arup Canada to create a tool to measure regional transit accessibility called WithinReach. It allows transportation professionals to quickly evaluate and compare existing services versus potential projects, using availability metrics. The calculation requires the summarization of all transit route operations across a geographic area into performance standards for one service day. In the Greater Toronto-Hamilton Area (GTHA), detailed timetables are available in the General Transit Feed Specification (GTFS) format, however manually preparing this data for WithinReach is labour-intensive. Due to the frequency of changes to vehicle and route schedules, and the number of regional agencies, maintaining an up-to-date summary of operations is cumbersome. This report covers the design, development, testing, procedure, and validation of a Transit Network Builder (TNB) to automate this effort. The TNB abstracts GTFS schedule data into performance benchmarks for use with the WithinReach Tool and the TransCAD transportation planning environment.