If it persists for a period of time, then it's probably a slow order (http://en.wikipedia.org/wiki/Slow_order). This is an impermanent decrease in speed limit for a given section of track.
Usual reasons for slow orders are deficient track scheduled for future maintenance. Simply put, the slow order is somewhat akin to the lower speed limit when driving through a construction zone.
However, certain sections of track have do have permanent low speed limits. The speed limit coming into Harvard Sq. on the outbound track is merely 6mph. This is due to the extremely sharp curve. All cars in the trainset must clear the curve before a higher speed can be taken, which is why people often wonder why the train is going slow before and after the curve (depending on where you are sitting).
It's probably a Slow Order
Usual reasons for slow orders are deficient track scheduled for future maintenance. Simply put, the slow order is somewhat akin to the lower speed limit when driving through a construction zone.
However, certain sections of track have do have permanent low speed limits. The speed limit coming into Harvard Sq. on the outbound track is merely 6mph. This is due to the extremely sharp curve. All cars in the trainset must clear the curve before a higher speed can be taken, which is why people often wonder why the train is going slow before and after the curve (depending on where you are sitting).