I did the commuter rail for 20 years, gave it up last year. I did the math on the total cost (purchase price, insurance, gas, routine maintanance) of a commuter vehicle vs. the rail pass (Zone 8) a few rate hikes ago, train was about 20% higher. I communicated the data to the MBTA, they agreed, suggesting convenience was the main driver for using public transportation.
I switched to driving last year (Pepperell to Cambridge), and bought an Accord Coupe V6. I schedule my hours to avoid sitting in traffic, and am not going to subject myself to an un-comfortable anemic vehicle for 3-4 hours/day. I get 28 MPG. By the time I retire, I'll have close to 140K on it. I get that you need a smaller car, but don't punish yourself ...