The longest trains that can be accommodated by most Swiss stations at that time was I believe around 14 UIC-X type coaches (26.4m each). At that time, there were still a number of shorter length coaches in use, so some trains could be longer. It was not uncommon to find trains made up of Italian, Belgian, French or mixed coaches with lengths from 22 to 25m each. Even the SBB lightsteel, EWI and EWII domestic coaches were shorter, allowing for longer trains. The station platforms at Milano Centrale were not that long and for that reason, many trains consisted of sections that were detached at Chiasso and only part of the consist went all the way to Milano. If I remember from my trip in 1971-72, there were a few longer trains that continued past Milano to Bologna, Firenze and Rome that were longer than the platform at Milano Centrale, but passengers for Milano were advised to take place in the first coaches.
I know that Luzern had two designated platforms (Tracks 8 and 9) where overly long Gotthard trains would stop, as I believe did Zurich HB.
The book "Die Gotthardbahn" by Marti and Trueb has some photos that show these long trains in the 1968, 1971 and 1971 editions (Orell Fussli Verlag).