Did trains #50/51 ever handle thru cars for Montreal to/from GCT?
Prior to the late 1950's, the through cars to Montreal were handled on #95 "Advance Empire State Express" and #55 "Laurentian". When these were discontinued c. 1958, #51 handled through cars via Albany / D&H on the daylight trip.
Eastbound to GCT, the through cars from the D&H were handled by #90 "Chicagoan" or #40 "Mohawk", both of which arrived several hours before #50.
You can check detailed schedules at Terry Link's "Canada Southern" website.