I've heard about such a machine, which is/was pretty much just a jet engine pointing down at the ground to blast/melt snow and ice away. Does that sound like this machine you saw? If so, it's been kicking around the property for a while...but it seems to be a rare animal. Apparently it also tends to fling the ballast all over the place.
Let me clarify a bit... I have seen, as in laid eyes on, jet snowblowers many times. But this was the first time I've ever seen one out and doing its thing.
Haven't been around those parts much lately, but there is/was a jet snowblower kept at Wellington Shop on the Orange Line, which they'd often park on a yard track in front of the building in clear view from the station platforms. I seem to recall another one at Cabot Shop that could be seen from Foundry Street in South Boston.
While we're on the Blizzard topic, ditto to F-Line about the Mattapan shutting down every time a flake of snow hits the ground. Should have sent that Type 3 plow that used to sit around down there through the same kind of extensive rebuild the PCCs got, or at the very least build another one of those Lead Sleds for the line. Right now, I believe they have zero snowfighting equipment assigned to Mattapan, except I think maybe they clear the yard with a bucketloader.