Since it's Mapbox I wonder if they are using the Isochrone API[1], or if this is more custom b/c it's subways? I agree an adjustable duration of trip would be neat.
If they were using the Isochrone API, the buffers around the subway stations would take in account the street grid vs a circle.
That's really cool that Mapbox now offers an API to do this. It seems like it also factors in elevation as well!