Are you going during rush hours? (The line otherwise has normal 60-120 minute commuter rail headway.) Since time *is* money for Zipcar, your one-way Zone 2 ticket to Woburn costs $3.50, or 23 minutes at $9/hour, will it be faster to change modes of transportation at Woburn than just get an extra hour of Zipcar? The tradeoffs don't seem so attractive to me.