They have lots of gravel roads in the country. Bill Daras posts some photos showing that the Silver Line tunnel past South Station is becoming a gravel road as well:
... If someone paved your driveway so that it felt like a dirt road, was flooded 24/7 and started to turn back to gravel after five years you would have the contractor rip it up and start again. In Boston you just let it go, and ask for over a billion dollars to do it all over again.