Bing Maps REST Services has added a transit travel mode so you can get transit routes and even display a transit route on a static map in your application. All you need to know is where you want to go and what time of day and then the REST Services will return a transit route based on your request.
To get a transit route, use the existing URL for getting a route and specify the travel mode as Transit. Then set the new dateTime parameter to a time value and set the timeType parameter to departure, arrival or lastAvailable and let the Bing Maps REST Services do the work. Transit route steps can include walking instructions as well as transit information.
Here's a sample: Walking to Bellevue Square Mall from the bus stop: http://dev.virtualearth.net/REST/v1/Imagery/Map/Road/47.615635,-122.20435/15/Routes/Transit?timeType=Departure&dateTime=3:00:00 PM &wp.0=Space Needle&wp.1=Bellevue Square Mall&mapVersion=v1&key=YourBingMapsKey