Why aren't mixed trains financially viable?

mtuandrew wrote:You might be able to get away with a mixed intermodal-passenger train on certain corridors, though woe be unto the passengers.

There was such a train: Three Rivers.

Amtrak tried this a number of years ago by running express cars and roadrailers on its trains. The experiment irritated passengers since trains were delayed due to adding or subtracting express cars from the consist, it pissed off the freight railroads since they thought Amtrak was intruding on their turf and it lost money. Nothing to see here. Move on.

It sounds like a stunt by a railfan-owner of a regional, or an Essential Rail Service in Canada.

The "Three Rivers" was the last gasp of the Broadway Ltd Washington section,it was more or less Amtrak's version of a "Mail and Express"
train between DC and Chicago,having more MHC's and Roadrailers then passenger cars(Amfleet I's).
Believe this was the train that started the Host RR's objections about the "Mail/Express " services and the speed restrictions
on the MHC's and Roadrailers were slapped on.

See also the Warrington era "Amtrak Mail and Express" program. It failed.