I'm guessing he meant the accommodation charge added to the rail fare.On Amtrak? I'm pretty sure the number of occupants increases the price. It did when I tried out a route on the CS.The price of any accommodation doesn't change with the number of occupants. So, I would say "book it!" for the family room.
You do like kids, don't you? Just in case the conductor tosses a couple in with you.
Most hotels consider two adults in the basic price, but beyond that many charge more per additional adult. A few low budget chains charge extra for a second adult.