Not when EG3 is finished, the 67s can saunter along at a ton in all the places the 168s will be able to.
I think the speeds will be 100 from Banbury to Wycombe probably the 50 through Wycombe will stay, then 100 from wycombe to ruislip then 80 to neasden, working it out it will hopefully knock 20 minutes off the times, this of course needs a clean run without catching up anything else, the 67's out accellerate the 168's by a country mile i suspect there will be a lot of train planning involved.
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WSMRs dont exactly dawdle along until South of Princes Risborough where they are limited to 70. There is no way you are getting 45 minutes out of it.
Current speeds are from Leamington, 90, short 75, 90, 75 through Banbury, 90, 65 through Aynho, 85 until Bicester where it is 25, then 100 until outside risborough where it drops to 90 then 60, then 70 to Wycombe, 50 through Wycombe, then 70 to S. Ruislip, 60 to neaseden, 40 through Neasden, then a short 60 then 50 until you reach the approaches to Marylebone.
The speed increases are above and also there will 90 from Shrewsbury to Oxley in places from December however this won't make a lot of difference because of the slot at Wolves and Stechford.