Mapnificent answers this question by drawing travel-sheds for trip lengths you select. They dynamic slider to change trip travel time is really cool. You can also change the settings to include biking to/from stations (rather than walking) and you can compute the intersection between two or more travel sheds.
I’d love to see them add bike and car travel sheds to this map. Presumably the data could be pulled from Google in the same way as transit information. Pairing this information with NAICS data could provide a really powerful accessibility map.
One drawback (at least for the Twin Cities), they don’t appear to have any data for opt-out transit services (Southwest, MVTA, etc).