A comprehensive graduate teaching assistant (GTA) training program in mathematical sciences designed at one institution is being adapted and replicated at two peer institutions. Using a case study approach, this paper outlines the development of the program components, which include a first-year teaching seminar, peer mentoring and support from a peer TA Coach, a Critical Issues in STEM Education seminar, and K-12 outreach to inform understanding of the pipeline. Additionally, adaptations due to institutional context and the impact of the COVID-19 pandemic are described. Implications for components of the comprehensive program, based on GTA-provided feedback, are discussed.