The Braintree branch of the Red Line seems susceptible to 'ten' codes on a regular basis because of poor track maintenance. Between dried and splintered wooden ties near some stations, and poorly designed versions of the concrete ties that have fallen apart because of the 'flat' wheels and constant pounding, the entire branch for the two years I have used it, has been prone to slow downs and creepy-crawling along. Robert loves to talk about 50 mph speed limits along the strtecth, but in the past two years, there has only been two, between Quincy Ctr & Braintree. Lately, I have seen them hit 50 between small sections between JFK and N.Quincy. It seems to be getting better! One can only hope!
(And I am sure others will submit the slow downs around the Cambridge side of things because of new sections joining old sections at impossible angles!)