This app visualizes the areas accessible via public transit in Greater Boston within defined time periods (15 min, 30 min, 45 min, and 60 min). It runs on a local instance of Open Trip Planner. Set up your own per the instructions on the MBTA's Open Trip Planner repository: https://github.com/mbta/OpenTripPlanner.
This app was adapted from the method outlined here: https://xang1234.github.io/isochrone/