The Colorado Railcar demonstrator actually tested service on the Princeton Branch with a Comet V cab car back in 2004, so that's a DMU.
(
https://www.youtube.com/watch?v=4rPNsy7qdbw)
I have been on the "Dinky" once and I feel that it is essential for the community since it is perfect for anyone, especially students at Princeton University and people who want to tour the town (I have been to the nice little town on numerous occasions) to travel from that region to a busy Amtrak and NJT station, so this way they can go from the town to the main line without renting a car.
But since this is an Amtrak discussion and seeing how we are talking about short operations, could it be possible for Amtrak to run or contract out someone to run short branch lines to connect towns to Amtrak stations? The rolling stock used for this cannot be limited to existing stock and coaches, and maybe they could use new small DMUs, Stadler/Siemens/Alstom two or one car multiple units, or even something known as a "very light rail"(
https://www.tdi.uk.com/en/revolution-vlr-consortium/).
"Look down, step over, and watch the gap!" - Dr. John Clarke, The Gap Rap (2010)