Upvote:4
The main reason for many Bakerloo line trains having an extra driving cab car in the train somewhere is because the current carriages/trains were not originally purchased for that line. The current stock is (refurbished) 1972 train sets which were Northern Line stock.
https://en.m.wikipedia.org/wiki/London_Underground_1972_Stock
Northern line trains were a carriage shorter
This page gives a longer overview of the historical rolling stock of the system.