Because otherwise this forum would be endlessly complaining about how not restoring it was "stupid", "short-sighted" or a "mistake".
Plus the actual timetable was not known at the design stage, and never good to rule out choices when the extra cost is relatively marginal (I.e. the track alignment would have to at least passively provide for the platform, so may as well build it whilst you're at it)
Platform 3 already existed, and was use once daily for an up peak service, and also during engineering work. It was very short (ca 5 cars) because of the intrusion of the Northampton bay at the S end (lifted ca 1965). Platform 4 still existed, too, (although with no track alongside it) but was completely removed and replaced.
But platform 3 (extended to 12 cars, like the others) can only be used by a down train that has run on the slow lines from Sharnbrook.
Plus, weren't the Slows doubled first and then used to divert trains whilst the Fasts were wired?
Didn't see that; the Up Slow only came into use very recently.
The slow lines have always been used during Sunday engineering work.
One naive answer would be that it allows a greater frequency to run when there is engineering work on the fast lines given the increase in expectations from travellers that the increased frequency engenders. The fast lines potentially need more attention with the increase in service.
But the low line speed on the slow line Bedford-Wellingborough (at least) would handicap frequency when trains are diverted from the fast lines.
I don't criticise the rebuilding of Wellingborough with 12 car platforms on all lines. My point really was; the slows were re-instated and electrified but did not have an increase in line speed, seems to be only 2 out of the 3 essentials Isn't increasing the line speed after all this work has been done quite difficult? Of course, depends how much 'future-proofing' was built in, but if the line speed is limited by alignment that would be better corrected as part of the re-opening of the line, surely.
I may be missing your point but the Corby line has been doubled north of Glendon. Along with. the re-doubling of Manton Junction some while ago there are now continuous Up and Down 'slow lines' all the way from Trent to London. A northbound freight can closely follow a northbound Corby terminator as it switches to the opposite line at Corby.
I thought that there was an electrified turn back siding just north of Corby that a passenger train can be shunted into (ECS) if it is booked to stand for an unduly long time before returning south.
I was mainly referring to the direct route to Leicester. The Manton line does indeed provide a slow line Glendon to Syston, but it's a bit of a long way round for poor old freight, and might transfer occupancy problems to Manton - Syston - which has freight to the E Coast ports and XC coming in/out at Manton from Peterborough
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I thought there was a policy to have the passenger service in the platform while it was at Corby; there's such an arrangement at St Pancras but there's a lot more passengers there!. Sometimes using the ex GC HST from Corby in the evenings, this certainly did scuttle off to the turn back siding if there was a freight about.
As an aside, there used to be a long low speed slack North of Harringworth viaduct (I suspect embankment slip). Is this still there?