We investigate the effects of mobile-sourced ridesharing via platforms like Uber, Lyft, and Didi Chuxing on the use of public transit systems. Our study uses trip-level data about Uber usage in New York City, turnstile data about subway usage, and trip-level data about taxicab and shared bike usage. We find that on the surface, ridesharing and subway usage are positively correlated. Exploiting ...