City of Everywhere probably held the record, but there were actual single trains which got close. In the eartly 60s around xmas the COLA and COSF which were normally combined were split--all first class one train up to 18 cars w/extra steam heat car on rear, all coach same, then re sorted at Ogden .
The seasonal Florida Special would have been 16 cars or more, and there is documentation of East or West Coast Champions 18-19-20 cars long in the 60s.