1961 March, “Talking of trains”, in Trains Illustrated, pages 136–137:
With their high horsepower, the new Bo-Bos, of which six are on order for trial purposes, but which should ultimately number 50, will take over most of the Gotthard passenger workings from the 6,000 h.p. Co-Cos, permitting the latter to take a greater share of the steadily increasing freight workings over this route, the aim being to work 1,200-ton freight trains up the long 1 in 38-40 approaches to the Gotthard tunnel with 6,000 h.p. Co-Cos in pairs.