So the two macro trends are a) cheaper energy and b) improved economic conditions in Maine. Cheaper energy has led to some loss (from peak) of commuters closer to Boston but improved economic conditions in Maine have meant increased use of the service for leisure travel and likely some increases in longer distance commuting as well...
I'll offer a different take. First, cheaper energy (read gasoline) would have more of an impact on the long distance modal choice. For NH commuters driving to Boston, if gas prices go up, say to $4/gal, their commute fuel cost goes from $10-13 to $16-20. Given all the other costs associated with commuting (tolls, parking, other vehicle operating costs), $6-7/day isn't a huge game changer. It's more about other choices, .e.g., driving to Haverhill or Newburyport to hop on MBTA. NNEPRA lost NH commuters due the service debacle a few years back. With service having gotten back to normal, those commuters have returned, and then some. In 2017, ridership at the six stations closest to Boston grew 9% over 2014.
Now look at the more distant stations.
Ridership at the five furthest stations from Boston declined 5% 2017 vs 2014. The longer-distance service got the ol' double-whammy in ATK FY15... fuel prices fell from the mid- to upper-$3s/gal to the $2s/gal in Q4 2014. Whammy one. Then service imploded. Whammy two. The gas price headwind has lessened somewhat, but remains. Service consistency has stabilized, and maybe even improved... service has even been added (east of Portland)... economic growth surrounding the corridor is pretty solid... ridership has clawed back from the nadir. Yes, I'm the service's #1 constructive critic, but it has to be recognized that the eastern half of the service has not fully recovered. There's a problem.
One hindrance to "eastern" growth is "western" growth. The more folks that buy EXE-BON seats, the fewer the seats available to folks trying to buy POR-BON seats. While anecdotal evidence, some Portland resident friends of mine recently told me they were going to a Bruins game. I asked if they were taking the train; they said it was sold out, so they were driving. I'm guessing the train was not full departing Maine, leave alone Portland.