the services continued to change for many years, especially when the tram lines over blackfriars bridge opened on 14 september 1909. during the 1920s the lcc also realised that for the subway to remain profitable it would need to be able to take double-deck trams. in 1929, it was then decided that the headroom of the tunnel would be increased to . this would be done by raising the roof or deepening the tunnel as appropriate. work started on 11 september 1929. the work resulted in the replacement of the cast iron tubes with a new steel girder-supported roof. the sewer was also diverted. in some places, the trackbed was lowered by . this meant that the walls had to be underpinned with concrete. after the last service ran on 2 february 1930 the tunnel was closed. it did not re-open until 14 february 1930 which was the formal re-opening. tram number 1931 was used on the new route 31. public services started on the next day. as well as the whole tunnel being rebuilt, the two tramway stations were as well. services now ran from hackney to wandsworth or tooting, leyton to westminster, highbury to waterloo or norbury and archway to kennington.