This would have been in the mid 1980s, on a weekday. The run definitely gained a lot of time against what it was booked to do. Wonder if it was book to make intermediate stops but they cut them out for some reason?
Edit: Just found some photos from the day; it was 5th April 1983. That was a Tuesday.
April 83 would still have been the first TT for regular HST deployment. So the semi-fasts were all still booked for Cl 45s, I think. If your train was a Cl 45 but running late, I suppose control may have decided to omit stops to avoid it holding up a following HST running to time. This would definitely explain how it could pick up so much time, yes. I have no idea if this did this regularly or not, but it would make some sense. (In the days of 2 TPH, all Cl 45s, they would divert the semi-fast onto the goods lines at Glendon North and allow the fast to overtake.)
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On the London-Leicester timings, my 1978-79 all-line timetable has:
The down Master Cutler from St P at 1720, arrive Leicester 1844, i.e. 84 mins.
Many down "fasts" during the day were xx53 off StP arriving Leicester xx30, i.e. 97 mins.
Non-stop? On a weekday? 97 minutes does not make sense at all. This is only 2 mins faster than the best steam timings. Unless there were some very heavy pw work planned in this period, 97 minutes would enable three stops to be inserted, or two if one was Bedford.
But if the Cutler could do it in 84 minutes, there can't have been any special PW work.
AIRI, in October 1975, to cut costs, BR axed the oh-so-effective 'integrated' timetable that had beenworking since 1968 or 69. (ie semifast, every hour, left St Pancras 30 or 35 mins before the fast, waited at Leicester to be overtaken, and thus provide a 1 TP 2 hours direct fast each to Derby and Nottingham, and 1 TP 2 hours "pretty fast" but with a change and stopping at Loughborough - ie 1 TPH fast to Sheffield, and more or less 1 TPH fast to Nottingham and Derby.)
So, the point is, once they abandoned this system, AIRI they had to insert a few extra stops into some of the 'fasts', at least during the off peak hours in daytime - maybe this is what you are referring to as 'fasts' in the 78-79 TT?
It's also possible that after this they made some consists of up to 10 coaches on the fast trains - but the extra 34 tonnes or so would maybe add 60-90 seconds of running time, it still not explain an additional 13 minutes.
The up Master Cutler was then 0823 off Leicester arriving StP at 0950, i.e. 87 mins.
And many up "fasts" were xx25 - xx06 or xx27 - xx09, i.e. 101 mins and 102 mins if my maths serves me.
Again, I have to ask, were these 'fasts' non-stop? these timings do not make sense for a Cl 45 + 9 - which c 1973 when ETH sets were introduced were no worse than 85 minutes, i'm pretty sure.
I don't have a timetable for the early 80s when the HSTs were coming in, but seem to recall about 90 mins for a non-stop Peak and 75 for a non-stop HST. It was really noticeable how much faster you'd top the summits at Desborough and Sharnbrook with an HST.
I must have some logs somewhere.
This would be where the power available with an HST would be most obvious.
Especially in the up, with the PSRs at Mkt Harborough and Wellingborough, a Cl 45 on a fast struggled to gain any meaningful acceleration up Desborough and Sharnbrook banks.
I can't remember exactly, but I think the best you could normally get would be around 58-60mph at the summits with a 45. I'd imagine HSTs can make 80 mph with little difficulty? (ANd I guess there may be a PSR on the top of Desboro because of the curve there?)