I don't know what all the hype is about anyway, it's not like those huge baggage cars don't have room for a couple extra inches on a bag.. <_<
Well checked baggage does allow for larger suitcases than what is allowed for carryon, 36x36x36 compared to the carryon limit of 28x22x14. The 50lb weight limit is as much about preventing one bag from crushing another in the baggage car as it is about preventing injury to employees who have to move the bags around. Railrookie may correct me, but I seem to recall that even UPS has a 50lb weight limit for normal packages. I think that there is a way to exceed that limit, both with special markings and extra charges, but again I do believe that a normal package cannot exceed 50lbs.
The smaller size limits for carryon are done in part to make sure that every passenger gets a fair amount of space, as well as again to prevent injury for both passengers and crew.
And when compared to the airlines, Amtrak's baggage policy is still far more liberal than any airline. Some now charge you for even the first bag, most charge if you go over two. Amtrak allows for two carryons per pax, and three checked per pax, before they start imposing additional charges.