I've created sotmg similar that shows where you can get to from any stop at any time, with closer stops showing as 'hotter'. This is useful if you are trying to find a place to live that is quickly accessible from work at the end of the day, for example. See it at www.transitheatmap.com.
Could you tell me more about how your accomplished this? I'd like to replicate it.
It's been 5 years but... I believe that I counted number of services calling at each stop using the GTFS imported into a database. Then I plotted each stop on a map using the coordinates provided in GTFS, and brought that into the Google Maps heatmap library (of 2012) with the intensity set by the number of services calling at the stop.