Construction started, the tunnels through central London being expanded and realigned and the stations lengthened, but it proved impossible to modify Wood Lane station to take 8-car trains and a new station at White City was authorised in 1938. The line was converted to the London Underground four-rail electrification system in 1940. The positive outer rail is higher than on other lines, because even after reconstruction work the tunnels are slightly smaller. Most of the tunnels for the extensions to the east of London had been built by 1940, but work slowed due to the outbreak of the Second World War until eventually suspended in June. The unused tunnels between Leytonstone and Newbury Park were equipped by the Plessey Company as an aircraft components factory, opening in March 1942 and employing 2,000 people. Elsewhere, people used underground stations as night shelters during air raids. The unopened Bethnal Green station had space for 10,000 people. In March 1943, 173 people died there when a woman entering the shelter fell at the bottom of the steps and those following fell on top of her.