In a two-year time span, 2013-14 through 2015-16, the National Writing Project's College, Career, and Community Writers Program (C3WP) achieved significant results in an independent, intent-to-treat, randomized controlled study. Middle and high school teachers in high-poverty rural districts in ten states changed their practices in response to the program, and their students outperformed peers on a rigorous measure of source-based argument writing. Since then, the NWP has made use of subsequent federal grants to scale up C3WP activity through teacher leadership institutes and programs in high needs urban and rural schools, and has extended the program to upper elementary grades. C3WP programming has scaled to 96 local NWP sites in 44 states within five years from its launch. What made this combination of high quality, effectiveness, and scalability possible? Drawing from research on the organization of educational improvement, we argue that the National Writing Project is an improvement infrastructure that continuously generates educational improvement capital, enabling it to solve important problems of practice at large scale.