wrote

Have they always used Tube stock on the Island Line since its
electrification in the 60s?
Yes - when it was electrified in 1967 they used 1926 stock, which had come
from the Piccadilly Line. This was replaced in the mid-1980s by the current
1938 stock.

The 1926 stock was formed, on the Island, into 4 car sets with a driving car
at each end, labelled in SR tradition as 4VEC, and 3-car sets which IIRC
only had a driving car at one end and labelled 3TIS. On Summer Saturdays the
holiday traffic required a 7-car train every 12 minutes - and there was a
seprate service of petrol-driven trams between Pier Head and Esplanade.

Peter