South Coast Rail: at what cost?

I have a family of 4. I found out on Friday that one of my son's favorite athletes would be at the Seaport World Trade Center in Boston this weekend. Great, I thought. The kids love to take the train. Let's make a day out of it.

So I went over to MBTA.com to check out what it would cost. Mansfield is in what they call Zone 6, so an adult fare is $8.75 each way. Kids under 11 are free. Ok, so that leaves 3 of us X $8.75 X 2 = $52.50. Wow, over $50 for all of us just for transportation. 

Contrast this with driving up & back. 72 miles round trip. Assume 20 mpg & $3.29 gas and you end up paying less than $12. With a $40 margin to play with, I think I'll take my chances looking for one of the many parking meters near the Seaport World Trade Center. 

All of this brings me back to South Coast Rail, the state's planned expansion of commuter rail service to Fall River and New Bedford. Recent estimates peg the cost at $1.82 billion; less (a mere $1.27 billion) if diesel locomotives are used instead of electric power. How is all that going to impact train fares? 

At what point does expansion of rail service within the current environment at the MBTA become cost prohibitive? If it's already uneconomical for a family of 4 to ride the train to Boston for a day, who is this service being created for? Commuters lucky enough to get a commuter rail pass as a work perk? Politicians who hope to have a train station named after them? Well, not to worry, I'm sure they'll just find a way to tax drivers more to feed the beast.