I was recently looking through old Trains
"All Diesel Issues" and they indicated that one B&O loco, 56 (and 56x) worked the Capitol Limited from Washington to Chicago and return (alternate days in each direction) for a full year without missing a run. This was two units with four engines, but they were the older 12-201A type. Santa Fe carried travelling maintainers who could work on an engine while the train continued on the other three, and I assume B&O had a similar arrangement.
Even so, 56 must have been a "good" locomotive where everything worked as intended. No similar claim was made for the locomotives on the "other" Capitol Limited, that ran the matching service on alternate days for example.