Isochrone is the area accessible from a point within a certain time threshold calculated using the actual street network. A few years ago, Baidu Map conducted a study that compared the theoretical services area (500-meter buffer) and actual service areas (500-meter isochrone) of bus stops in Nanjing City (See image below). The study overlaid the isochrones on top of the buffers, which gave people a clear idea on how well the bus stop locations connect to the adjacent blocks.
I got curious and replicated this method on all the MARTA train stations. Instead of 500 meters, I used a quarter mile as the walkable distance in the US. I used OpenTripPlanner to calculate the isochrones, and mapped the isochrones, the buffer, the train stations and alignments on the map using QGIS, and published the map as html using Qgis2web tool. Below is the result: