Looks like an issue with one of the Providence train sets tonight. Train 823, the 4:53PM outbound train (which was using a set with control car 1800) broke down outside of South Station, so they backed into South Station and had to switch to another set. I think they went from 8 doubles to 4 doubles and a single.
They then try to use that same set for train 833 (7:30PM outbound), but it was still having issues and again the train had to be backed into South Station after departing to switch for another set. They have a couple more chances to get that set down to Pawtucket, otherwise either 806 or 808 tomorrow will be shafted with a pretty small set (both of which are 8 car trains).
From what I remember the Pawtucket layover usually has two 8 car sets (806 and 808), a 6 double car set (810), and then whatever's at hand for 800/802/804 (usually the 4 double/2 flat sets).