One of this week's postings on the Diamond Geezer blog may be of interest: http://diamondgeezer.blogspot.co.uk/201 ... ilway.html
It's about the new automatic signals installed on the Whitechapel & Bow Railway (now part of the District Line) at the time of its electrification early in the 20th century. It includes plans of the proposed works and extracts from the Board of Trade's inspection and related correspondence, as found in the National Archives at Kew.
The Geezer writes on his blog daily, usually about London, often about railways. But I don't recall him posting anything so detailed about signalling. He lives in Bow, so Bow Road is his local station.