Don't know whether this should go in the Packing forum or Beyond Europe, but here goes.
I guess I assumed that an "underseat" bag that would fit under the seat on a 737 would also fit under the seat on a train, talking Amtrak here.
But I look at their Carry-on policy and it is
1 personal item, 14x11x7 inches, 25-lbs
and
2 carry-on items, 28x22x14, 50-lbs each.
The personal item goes under the seat and the carry-on either in the overhead rack or luggage storage at the end of the car.
14x11x7 is smaller than most rolling underseaters. They usually run 16 to 18 inches width and height and 8 to 9 inches deep.
So is the underseat space really smaller on an Amtrak train than on a 737 plane?