There is a 'dance' of 3 Amtrak trains have to perform at Springfield between 2:45 and 3:30 PM, give or take, when 55,56, and 449 are on time...or close to it. Throw in a CSX or G&W freight crossing the diamonds about the same time, and it becomes a game of chess between 3 railroad dispatchers controlling traffic to and from the diamonds. I think CSX is the 'owner' and thus has the final say so. If there aren't any freights coming through, what is supposed to happen is that the southbound Vermonter (on Guilford trackage) goes straight across the diamonds and stops about 150 yards beyond, contacts the Amtrak dispatcher (Conn River Line south of SPG is owned and dispatched by Amtrak) who then lines up the reverse move to take the train into the SPG station on the south-most track, closest to the Amtrak Station (have they moved yet to the newly refurbed station? As of 2+ months ago, it hadn't happened yet). Depending on how on time the northbound Vermonter is, they run him directly into the station on the next track over. And if they are REALLY lucky...448 from Boston will come through and stop on the mainline to discharge and load its passengers. As the southbound Vermonter was backed in, once the Amtrak dispatcher clears the route and gives them the signal, they leave in the forward direction and keep going. The Amtrak dispatcher has to arrange for the northbound Vermonter to back out and down the Conn River Line to clear the switches to go north. Then the CSX dispatcher has to give permission once again to cross the diamonds, and the Guilford dispatcher set up to 'receive' the train.
So, why is the southbound run so slow into Springfield? I'm guessing they passed a usually yellow signal on the north side of Springfield indicating the lack of authority to cross the diamonds ahead. As the diamonds are on a slight curve for north/south trains, it's probably limited to 'restricting' speed. It could also be related to where the northbound Vermonter is at that point in time...it could be backing out onto the second track on the Conn River Line and will have to wait until the southbound clears the single north/south track across the 2 CSX main tracks. Fortunately, there's 15-20 minutes worth of padding for both Vermonters at SPG, so a slightly late or slow approach to SPG will still get an on-time departure.