I actually think the figures on the other Old Colony branches are worse in terms of % of decline in ridership. I think with the Greenbush line, it's a bunch of things. Remember it opened in 2007, right as the economy was heading south. And I believe that was also during the first of those well publicized delays."the "silent strike", the Worcester train that took 4 hours to get home, etc). One other thing that's been probably discussed is the train frequency on the Greenbush line is terrible. But I don't know if that can be fixed or not. Can it recover? Maybe. I'd like to think at least the line is preserved for the future. If after it was abandoned we decided to encroach on the right of way, then in 20 years we needed...whooops....
I think too, even someone like me who likes trains and supports, sometimes taking the car SEEMS cheaper, easier, and faster. Notice I said SEEMS. Because you don't have to fill up every single day (though you may have to pay for parking), but you might have to pay for a ticket every day AND subway transfer. And when the fares and passes went up. Although I dunno, even though they ARE expensive, the passes do include subway, bus, and some include ferry service. I know at least for the NYC metro area, MN, LIRR, and NJT don't do that (I could be wrong though). So it still can be a good deal. (Primarily though if you work away from the two terminal stations). If the math does actually work out in the commuter rail's favor, maybe they should do some "do the math" ads where they show REAL numbers.
For me, there's an illusion that you control your destiny when you're in a car. You're on a train and it gets delayed, nothing you can do. There's a delay on the highway? I'll take the back roads (even though it will probably take the same amount of time). Again, the keyword is SEEMS. And yes there are times when driving is faster, in some cases A LOT faster than taking the train, not really anyone's fault, just the way it is. Keolis had those "case studies" in their proposal. If they follow through with thinking about creative ways to attract ridership, maybe that will help.
