Wellingborough Sth to Barrow on Soar to be precise.
From what I understand* BR dropped a ball when they opted for 3aspect (seldom used) bi-di rather than 4 aspect and no bi-di.
I believe the bi directional wasn’t even allowed to be used for the first few years after installation. And now it rarely is unless a fright fails somewhere.
*I hope someone more knowledgeable will be along shortly!
The submission for authority to proceeed went in in May 1983 for a scheme that had been largely finalised in the latter part of 1981, having already been stripped back from first ideas of 4 aspect and then second thoughts of 3 aspect with slightly more extensive track facilities than the final version (and 50 mph crossovers instead of 40!) . It was at this very late stage in 1983 that the Market Harborough / Great Bowden re-alignment was dropped. The plans submitted involved as a basic scheme the continuation of 4 aspects to Wellingborough and then 3 aspects except for Kettering North Jn to Kilby Bridge which was to be 2 aspect, even though this would mean no future speed above 100 mph. Something actually added back in was 3 aspect throughout from Wellingborough to Loughborough. Full bi-di signalling on the 2-track sections was chosen as an add-on over the "minimum acceptable alternative of slow speed facing crossovers every 5 miles" for an extra cost of £40,000 over the cost for the simpler scheme of £445,000. Signalling was to be immunised to permit the eventual electrification of all routes. Interestingly, the argument for keeping at least one goods line at Sharnbrook was pretty well exactly the same as the argument for building the deviation a century earlier! There were to be two significant re-modellings for higher speeds, at Wellingborough for 75 instead of 65 to cost £21,000, and at Market Harborough for 90 instead of 60/75 to cost £1,614,000.
You're right about the use of the bi-di at first. The only bit which was used from the start (and was quite regularly used in ordinary service) was Leicester to Wigston. The rest was not authorised until a patrolman's lock-out had been developed, and that did indeed take two or three years to happen. After that I remember regular use of the single-line facility during engineering works and also on occasion during normal service hours to get round a failed train. Re-signalling schemes on the ECML and on the Western went in for SIMBIDS rather than the full bi-di (same speeds and same capacity on both lines) that Leicester got. (Indeed, is Leicester one of the biggest full bi-di installations on the British network?)
