What does "more like a business" even mean here? Historically Amtrak and the USPS budgets have been subject to political push-pull, so here I agree, both institutions need stable budgets. Other than that though, Amtrak can never make a profit because its competition, automobiles and highways, are subsidized by the government (very few toll roads exist in the US, it's mostly combination State and Federal funding which builds highways.) The USPS might be able to turn a profit but it would be really difficult given the long (probably unprofitable) routes it's required to carry and its inability to control its own rolling stock and employ certain workers means it would probably be a lost cause.
If you mean an eye to efficiency and a buffer from the changing of governments, that I agree with.