Buffers of public transportation transit times

I also agree this is an interesting problem. Instead of buffering transit lines though, I'd start out with stops, which are part of the Google Transit Feed Specification. RTD's feed is here. Perhaps use GraphServer to build trees.

I tried something similar once (using FME's NetworkCostCalculator).

The interesting part is in the visualization. We wrote the data to 3D PDF, with time as the Z axis, and a background image at certain time intervals.

You just turned on the 10min image, or 15min etc - to show how far you could get in a certain time.

And if you turn the network into a cost surface it looks like this: