I honestly don't think this will happen. It seems to be a popular idea with users of this forum but the numbers don't add up.
People on here seem to be under the false pretence that the Corby services cart around fresh air. Perhaps at stations north of Bedford, but from Bedford and Luton they are crammed.
How they think you'll fit those passengers on to already rammed 12 car train I don't know.
If they ran more than 4/5 coaches it might help. Surely the answer is to run longer trains on EMT and stop some of them at St Albans.
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And Cricklewood, Hendon, slow lines platforms at Elstree, Radlett and (I think) the stations north of Luton save Bedford.
All the stations north of Luton have 12 cars except the fast line platforms at Harlington (and possibly Leagrave I can't remember) and platform 4 (down fast) & 1a (Bletchley bay) at Bedford.
Kentish Town, Cricklewood and Hendon have 8 car platforms on all lines still. Mill Hill has 12 car platforms on all lines as it was one of the first to be done and they hadn't yet realised at that time they could save costs by not doing fast line platforms with no scheduled services or slow line platforms only served by the wall of death (Sutton loop) that will only ever have 8 car trains because doing Kentish Town and Tulse Hill is too difficult.
West Hampstead, St A, Harp, Luton A and luton (all 5 platforms) are all 12 car all platforms.
Elstree/Radlett slow line platforms only.
I did read a proposal at one point to not do 12 car Flitwick fast lines but I recall that was dropped after local commuter association intervened. Similarly I recall the post blockade timetable was going to have the limited stop trains fast to Bedford from Luton until the BCA saw the timetable and pointed out omitting Flitwick was not wise.
I think City is actually 12 cars plus loco because BR had ideas about running intercities through Thameslink in the late 80s when it was built.
Doh, I've been travelling on that line too long (two years more than you have!) and have even travelled on the down goods line on a Thameslink. Got looped in there one day just beyond West Hampstead when an EMT train failed in front, then back onto the down fast at Cricklewood.