I think we could all benefit from a local train line having a stop within about a half mile of our house
A half mile was choose as that's brought the fartherest people are willing to walk to get somewhere comfortably as a result, if we want to build a metro or regional rail system, then we as planners want to concentrate as much density as possible within that half mile radius for the maximum amount of riders.
And of course to distribute the benefits of living near transit stops to as many people as possible