(Replying in a new thread to accommodate extended waffle irrelevant to the original.)
This Is Complicated
The route was originally constructed by the North Midland with two tracks, one medium-sized tunnel, and one LBV, of 13 arches, at 179 miles 43 chains. [LBV: Lord Blitherwit's Viaduct; a subclass of Lord Blitherwit's Infrastructure Feature, which is the class of items of infrastructure built in an unnecessarily complex/expensive/awkward manner to try and get some [k]nob with a lot of money to pack it in making a pain of himself. See also: LBT, Lord Blitherwit's Tunnel, as at Haddon, where there is so little need for a tunnel that in some places the earth is falling off the top of it; or LBC, Lord Blitherwit's Curve, at Saxby, which gave rise to an interaction between workarounds and geology that ended up rogering more of his estate than he'd have lost if he'd just let them get on with their original plans, arf arf.] By the 1880s, the Midland were finding that this was no longer enough, and added an extra pair of tracks - except over the awkward stretch with the tunnel, and one or two other minor holes that got filled in later. The remaining two-track section extended from a bit south of the LBV to a couple of miles north of the tunnel.
Once WW1 was out of the way, they got on with finishing that remaining stretch, although the Grouping came along while they were getting going and the Midland's enabling Act became part of the LMS Act of 1923. The ground around the tunnel was dug and blasted out to form a great big cutting, while the main line through the tunnel was still live with trains running on it, with only the minimum of actual closure to knock down bits of the tunnel structure itself - indeed I'm not sure that they didn't erect shielding within the tunnel so they could take it apart with the trains still running; and the remaining bridges, junctions etc. were also adapted to take four tracks.
It appears that there was a bit of argument over some of the bridges with the local council trying to get something for nothing; they were saying "when you rebuild these bridges, we want them made wider so we can widen the road underneath", to which the railway said "but we're not rebuilding them, we're just building an extra bit alongside and leaving the old bit as it is". What ended up being specified in the Act was a kind of compromise, where the railway were allowed to do that on condition that if they ever did need to rebuild the old half, they must get rid of the "centre pillar" when they did it - apparently the idea was to split the road either side of the pillar until that was done. The Act specifies various bridges to which the "centre pillar" condition applies, and one of them is the LBV.
What was actually built for the LBV, however, was such that the idea of a "centre pillar" does not make sense. Only two pillars remain visible, and the road runs, with some constriction, through the full arch in between them. Of the two arches either side of those pillars, only half the width is still exposed. Two blue engineering brick abutments, with concrete transoms, extend across the full width of all four tracks; on the east side, where the new width was added, they support a steel girder span carrying the slow lines; on the west side, although the transoms are continuous with the east side and are all ready to carry a steel girder span should one be installed, they don't appear to support anything. The centrelines of the undersides of the two half-arches are some distance above the level of the transoms, so the original stonework vanishes unsupported into a mysterious dark void and what becomes of it within "noo-o-o-obody knows". [TODO: visit the location with a camera and torch on a long pole and see what dark secrets are thereby revealed.] The rest of the LBV, beyond the faces of the abutments, is entirely concealed beneath new fill, Lord Blitherwit himself now being too dead to object to its replacement with a conventional embankment plus bridge.
There is some suggestion that objections were raised to this construction on the grounds that the absence of a "centre pillar" precluded the possibility of splitting the road around it to gain width, so it wasn't strictly legal; however this faded out through a combination of fait accompli, the splitting idea not being a very good idea in the first place, and everyone getting sick of arguing over something so unimportant anyway. The LMS dug their heels in because they were fed up with the way the project was dragging on, apparently partly at least due to the contractor on this bit not being really up to it, and they wanted to get it over and done with; the council realised it was going to take forever even if they won the argument, and let it drop.
The entire area became increasingly badly affected by mining subsidence as time went on. All the lines crossing the coalfield were affected, the S&K just as badly as this one, if not worse. In 1955 Lord Blitherwit's house had to be knocked down to prevent it falling down of its own accord, because of all the holes that had been dug underneath it.
BR's project in the 60s to concentrate all Leeds services into a single station inevitably resulted in Central, having awkward unintegrated approaches and not being very "central" to the other two, getting the chop; but this raised the problem of how to get the prestigious expresses from the ECML into City without them getting tangled up with other services in the mass of junctions to the west of the station, as would happen with any straightforward reconnection of the approach of the GN Doncaster route. The solution was to additionally connect it in a less straightforward manner to the Farnley viaduct, giving ECML trains their very own approach to the station free of conflicts. It was, however, a rather roundabout approach - more or less inevitably - with a sequence of sharp, slow curves and junctions extending quite a distance out from the station, and only two tracks; the GN line itself also had only two tracks, and some significant wiggles and upsy-downsy bits. Nevertheless, it did the job as far as expresses via Doncaster went.
The Midland had always had the best route into Leeds of all the companies arriving there, being engineered by George Stephenson in accordance with his principles of minimising gradients and achieving the easiest possible run, and having had the early builder's advantage of being able to bagsy the best route. By now, also, nearly all of it had four tracks. It was in all ways a better alignment than the GN route, even in the days when that ran straightforwardly into Central.
In 1967 some knob called Arthur Dean decided, for no obvious reason beyond some eclectic and obscure ideology, to divert services off the Midland route, and instead run them up the S&K, with Leeds services diverging at Moorthorpe to share the two tracks and tortuous approaches to Leeds of the GN route with the Doncaster services. It didn't speed anything up; both routes were beset by subsidence slacks, and it introduced some significant extra junction/curve slacks on top. It also failed to make the best use of the already inadequate routing possibilities on the approaches to Leeds. It's not obvious to me what it did do, but it happened anyway.
The next "event" was that a chap called Noel Proudlock did some deep research into the relationship between mining operations and their subsequent effects on the running times achievable by trains going over the top of them, and came up with a system whereby, through liaison with the NCB, the duration, severity and extent of the speed restrictions required could be scientifically and accurately predicted in advance. He then set about analysing lines in the area according to this system. He discovered that a great many of the subsidence slacks were no longer necessary; the underlying seams had already been worked out, and the persistent rock'n'roll through the slacks was due not to continuing subsidence, but to nobody realising the subsidence was completed so now you could stop bodging it with ash ballast and get on with fettling it properly without all the work being wasted. He further calculated that if the appropriate permanent fettling was done on the Midland route, the timings for trains between Sheffield and Leeds/York could be significantly reduced. Consequently, from 1972 trains were gradually diverted back to the Midland route.
It did speed things up a bit, but it didn't work as well as it should have done and remained more "theoretical" than it should have been. Some trains were turned off at Oakenshaw to run via Wakefield, so they missed out on a goodly chunk of the advantage; and all trains suffered from the failure of the permanent fettling to be executed with promptness and dispatch. A good deal of it had been done in time for the new routings, but several significant slacks still remained, sprinkled along the route so as to preclude much useful acceleration in between; and dealing with them ended up as one of those things that drags on and on and on.
This wasn't originally supposed to happen. In the early/mid 70s the eventual arrival of the HSTs began to be anticipated, and routes identified on which - once there were enough of them - their characteristics could be used to advantage. The Midland route with its easy gradients and broad curvature was a particularly promising-looking candidate, with line speeds over 100mph or even 115 possible over much of it, albeit probably not the full 125. However, at this time the ECML still went through Selby. By the time there were enough HSTs for them to be used for this kind of route, the Selby diversion had been built, and especially with the reinstatement of the Mexborough curve it became just as quick for the York HSTs to blast down the ECML to Doncaster like Meat Loaf and turn off for Sheffield there, so the Midland route's potential advantage was superseded.
Somewhere around this time also the route suffered from the attitude of a Deanite chief civil engineer. Apparently this bloke took it upon himself to discard all the detailed scientific prediction that had been undertaken of the subsidence expected from the NCB's current plans in favour of making up his own narrative out of whole cloth, and painted to some critical meeting a picture of 13ft of subsidence occurring under a big river bridge. There was no evidential support for such a prediction, but apparently either nobody at the meeting actually knew that or else they were just too nesh to say anything, and so the black mark against the route was made part of the books.
Then when "sectorisation" was imposed it provided a whole new bunch of opportunities for bits of the railway to be clobbered by the fallout from dumb squabbles over who pays for what bit and who can wiggle out of it, instead of treating the railway system as, you know, like, a system. The S&K route still wasn't any better - it wasn't built as a fast route, and the fettling of its expired subsidence slacks had been similarly patchy and incomplete - but its existence alongside sectorisation enabled the Midland route to be killed off in pursuit of some artificially-created accounting fiddle and the inferior alternative used exclusively.
And hanging over all this, throughout the period in question, was the spectre of the cowboy on contract number two, fifty years before. Or perhaps lurking chthonically under would be more accurate. What appears to have happened is that, as described above, they converted the LBV into a bridge plus embankment by tipping fill over the ends of it to bring the existing embankment up to the new abutments, without doing anything to ensure adequate compaction of the material under the arches. This left the masonry structure still bearing load, but entirely inaccessible for any kind of maintenance. And the inevitable result began to make itself felt around the start of the time under consideration.
BR seem to have been remarkably incompetent and clueless over a very protracted period about dealing with it, and it's hard to get a proper handle on exactly what the problem was and how it developed; more questions seem to arise than are answered. By 1972 it had already got itself its own name: the "Chevet slip", a search term for which Google currently returns precisely one result but will henceforth return two. It was considered the only one of the slacks that remained unfixed in 1972 that was likely to carry on remaining unfixed, but I get the impression that at that point they had yet to determine exactly what was causing it. It also doesn't seem to have been horribly bad yet: the fast lines were still open, and there was only a 40mph slack from 179 miles 27 chains to 179 miles 40 chains - which itself is slightly puzzling, since that's still 3 chains short of the LBV itself, and with the thing only having 13 arches and the bridge bit being in the middle, the slack seems to be just off the end of the concealed structure rather than over the structure itself.
Next we have a few postings around the internet from Michael Kaye (MK55A in various internet railway contexts), which are quite informative. His description is of the collapse of the invisible stonework as a sudden and catastrophic event, which had everyone freaking out and closing the fasts to find out what had happened; it also comes over as being something he remembers personally as happening when he already knew the route, which would put it a bit later than 1972 as he didn't start until 1973. This suggests that whatever they were doing about it in 1972 not only didn't work, but didn't even get as far as identifying the cause of the problem, which then a bit later on suddenly got much worse and found them still needing to find out what was up (or down).
It looks as if when they did figure out what it was, they threw up their hands in horror, gave up, and found a further reason to discount the route. They ended up first closing the fast lines and crossing everything onto the slows to pass that point, then they took the fast lines away, then they started routing everything along the slows all the way and allowing the fast lines to go derelict.
http://www.youtube.com/watch?v=XVQmkkd0bOM provides another interesting and puzzling bit of personal reminiscence. As they pass that stretch of track there is a pause in the merry party's discussion of their in-cab vile flatulence contest, and a Scottish bloke can be heard saying "this is where the banking collapsed, I remember that". He then talks about driving A4s and being diverted off the ECML, and I think he's saying that that's where he remembers it from. If so it would imply that it was already well known that there was a serious problem on this stretch, long enough ago for it to have influenced matters right from the start; and raises the question of why they hadn't been giving the serious problem serious attention long ago too.
It may not be significant, but the point at which he makes his comment is also significantly southward not only of the LBV itself but also of the south end of the 1972 extent of the slack; it appears to be about the location of the bridge over Haw Park Beck. It's probably just when he decides to speak, but it has to be said that the edge of the embankment does look more meagre there than at any other point nearby.
I think that pretty much covers what I know about it so far. Unfortunately I can't find any more whatever it was that I found originally about the arguments over widening the road or the dodgy contractor. At some point I want to go and look at the archived original plans for the final widening project and see what clues they give me. Most informative of course would be to get my hands on BR's maintenance records for it, but I guess that might be a bit too much of a hope.
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