I am aware of the issues with mining subsidence on the MML, particularly on the missing closed sections, but how significant would the infrastructure work to remove these psrs have been
I am not a civil engineer, but when the ground is collapsing beneath the p way because of mine shafts hundreds and thousands of feet below, I'd say it would cost many, many millions (even at 1970 prices). In fact, it might simply be unsolvable, given the risks involved. Hopefully, some of our civ engineering experts might enlighten us more.
and how much of it has since been done when the line speed was subsequently raised?
Again, not a civil engineer, but what I suspect is that the subsidence problems largely solved themselves by natural collapsing over time. The civil and signalling engineers would then get to work to raise the line speeds at "normal" costing rates applicable at the time.
I believe some sections are now 110mph or 125mph aren't they. The GWML fast lines required some significant amounts of slewing e.g. at Twyford, but incidentally the relief lines on Brunel's original alignment didn't. Wasn't any of the MML built to 4 tracks from new?
ITYWF NO lines were ever built as four track routes. We had a thread on this issue some time back. (Remember, the railways were built as private ventures. Very few companies, if any, would invest in four-track routes before 'testing the waters' with a two-track route. Private capital does not take such massive risks.)
But the answer to your question regarding the Midland, no, I'm 99% sure none of it was ever built as four track from new (other than, perhaps, short sections built as junctions,or, possibly, short sections built for special purposes, such as to hold banking locos - but that wouldn't really constitute a four-track section as you mean it).
The initial investment in modernising the ECML with Deltics e.t.c. was very impressive and the MML does of course not serve Wakefield, but if it had received the same level of upgrades, would it still have been slower?
Almost certainly slower, yes - assuming you mean by 'same level' of upgrades you are measuring that in money terms.
Obviously, if you spent billions and billions to iron out all the PSRs on the Midland, including running through Leicester and Sheffield at something like 100 - 110 mph
, the it would come down to a simple formula of distance/velocity, and if the Midland route is shorter (I doubt it is, but it might have been) you could beat the GN route by a matter of minutes, or possibly just seconds. But that belongs to a fantasy thread.
The Midland did (and does) serve Normanton, of course - I don't know how much the good burghers of Wakefield consider(ed) that as a sort of "Wakefield Parkway" station.
