If you work with transit data, you might want to take a look at the following blog post.  Some great suggestions:

• Use services areas around transit stops instead of buffers around transit lines
• Incorporate frequency, like trips made per hour
• Consider travel time between places throughout the day
• Factor in transit frequency
• Incorporate demographic data to find what groups are underserved by transit or have a high level of need for better access to certain types of destinations

Check it out.