I was on a HST that had a failed powercar on departure from Edinburgh. We weren't even late enough into Kings Cross to make a delay repay claim.
I must confess that I didn't believe you. That is not to say I thought you were making this up, but that - assuming we are talking about a regularly timed 125 HST service (and not a special Sunday working with added recovery time, or some such) - somehow you were mistaken, and the second power car cut in for at least part of the journey without you knowing. Or stops were missed out to help the train recover some lost time which you didn't notice.
Because a) even under ideal operating conditions, an HST with a single Cl 43 power car of 2,250 HP simply could not but lose over an hour Edinburgh - KX by the basic laws of physics and
b) in the real operating world any such train would be looped at least two-three times so as to avoid a queue of ever later HSTs following behind. Looping the train would inevitably lead to further delays, of course.
But, before positing any such scenario, I set to work on a back-of-the-proverbial-fag packet calculation.
If we take the current Highland Chieftan timings, it takes 4hr 17 minute (or 4.28 hours) EDB-KGX. That's an average speed of 91.7 mph.
My own feeling would be that at best, an HST on half power would manage 75 mph average speed.
If so, any such train would take 5.24 hours.
So yes, if that calculation were to be correct (and it is only a guestimate, of course), an HST with one power car out could make ot to KX with about 0.04 of an hour (2.4 mins) less than 60 mins late, ie about 57.5 minutes late.
I still find it difficult to believe your train wasn't looped, which would have added to the delays and surely pushed the total above 60 mins, but, by my own rough estimate, it might be possible.
Of course, the huge unknown here is what average speed an HST on one power car can manage on such a run. If we assume an averge speed of 73 mph, ie just 2 mph less than my initial estimate, the timing jumps to 5.38 hrs, ie about 5hr 24 mins, and you could easily justify an hour's + delay.
Whatever, perhaps someone out there has a real log of an HST on the run with a power car out that could confirm (more or less) these figures?