This is a map based on data about how many people aged 16 and up take public transit to work by congressional district (110th Congress) from the 2006-2008 American Community Survey. As you can see from the legend at the left, the darker areas have higher transit ridership. This map was generated automatically under the "Geographic Comparison Tables" link.
If you haven't seen the Census Bureau's website, I highly recommend it. It's fascinating.