Very Short Amtrak Train

I was researching short Amtrak trains -- some of the midwest service trains, the Heartland Flyer, and the Borealis qualify, as well as the Portland-Seattle sections of the Empire Builder, and the Boston section of the Lake Shore -- when I ran into this video of a not-quite 448, the Boston section of the Lake Shore, leaving Rensselaer.

The narrator doesn't explain what's happening very well, and I had to poke around to try to get some idea of what's going on here. We know that 448-449, the Boston Sections of the Lake Shore, were discontinued in July due to a sinkhole east of Rensselaer on Amtrak's Post Road Branch, which connects Albsny-Rensselaer with the CSX Berkshire Sub at Schodack to continue to Boston. The trains have been replaced by buses. I've read that in the past, Amtrak has detoured over trhe CSX Castleton Cutoff, but not this time. However, it looks like something needs to run between Rensselaer and Boston to ferry eauipment for the Downeaster, Vermonter, Springfield trains, etc, even if passengers aren't carried on 448-449.

I read via Wikipedia that CSX also uses the Amtrak Post Road Branch via trackage rights. I asked Chrome AI mode about this, and it replied,

CSX has rerouted its freight trains traveling between Albany and the east via its Hudson Subdivision and Berkshire Subdivision. The freight would travel south from Albany to Castleton-on-Hudson on the Hudson Subdivision and then reverse direction to continue east on the Berkshire Subdivision.

So my surmise is this is an Amtrak extra movement that replaces 448-449 to ferry equipment back and forth to Boston for the Downeaster and other New England trains, but so far, I haven't been able to confirm this. But if it is, it must be following a similar route to the CSX freight detour. Normally 448-449 seem to run with as many as four diesel units, plus cabbages on occasion, to perform this function. Whatever it is, it's a prototype for a very short Amtrak train.