Solves multiple problems:

• Eliminates the need to turn trains around at North and South station. This is a major bottleneck and reduces the amount of trains we can run through those stations, limiting our headways.
• Allows us to run trains between North and South shore stations. Worcester to Lowell, Lynn to Fall River, Fitchburg to the Cape, you name it it's possible if we have the North/South Rail Link and enough trains.
• Allows us to share trains between the North and South shore lines easily too. Currently our main option is the slow overnight transfer of equipment via the Grand Junction Freight railroad in Cambridge. We could easily handle a broken train by sending a spare from the south shore up to the north shore or vice versa.
• Gives us a new connection between North and South Station. This on itself could allow for redundancy in our downtown transit system which is helpful if subway lines need to be repaired.
• All of these things may eliminate the need for some passengers to transfer to the subway at all too. That allows for more capacity in our transit system and faster rides. That Lowell to Worcester route might take 3+ hours today since you need to ride 2 Commuter Rail trains and the subway. Commuter Rail trains have hour long headways at times and the subway may take 15-20 minutes at times between trains. Combine all of this and you may need to catch a 9am train from Lowell to catch a noon train to Worcester out of South Station to get to your destination several hours after someone who drives gets there. Why can't Lowell to Worcester take an hour or two via ONE train?
• And if we get more people onto a train of some sort, we get people out of cars and that frees up highway, roadways and parking for people who need or absolutely want a car. Some of these improvements could lead to a few thousand to maybe tens of thousands of new train riders. Maybe that's a few thousand cars off the roads.