Hmmm. Not much in it, methinks. It is true that, because St Pancras was designed to be built over storage space for Burton beer and crossed above the Regent's Canal that the start was much easier for engines. (Well, I suppose it still is, but we don't worry so much about gradients with the power available these days.)
But it was I think 50 or so to Kentish Town, and 70 to Silkstream Jcn. Then you had 80s at St Albans and Luton. From MP 34 near Leagrave to Bedford was fast, but that was because it was a long stretch mostly downhill.
Edit: oh dear - I've only now seen Yorkie's stern warning about new trains n stuff. I suspect this will be banished into the ether.
But that was on the down. I don't know what typical speeds were on the up in steam days. My guess would be a Scot or Jubilee would be doing well to top the bank at MP 34 at 60 mph, maybe even 55, after passing Bedford at speed. A class 45 would probably be doing around 75 mph.
