Bedford North Jn UFL>USL came out on 26 November 1978, with DSL>DFL two years later, on 1 November 1980. The relevant replacement part of the new Bromham ladder came into use on the same day. The entire job was completed with the provision of the slows crossover at the south end of the layout on 29 November 1981.
At the south end of the station the double junction close to the platforms, at Bedford South SB, had gone much earlier, t.o.u. on 12 March 1967, so that from then on trains calling at Bedford from the fasts had to cross at Kempston Road.
I seem to remember that in the up direction the UFL signal that would be shewing YY for the move into the station is pretty well at the 52¼ milepost, with Y at about 51½ and R at about 50¾, well away from the points. I think when I was travelling over that route regularly I agreed with your estimate that it was taking 90-120 seconds extra to stop at Bedford as opposed to passing on the UFL at line-speed. And then there was overtime in the platform, and then the very slow start from 3 back to the UFL ..... Ultra-tedious station to stop at!
Eh-up, there's summat queer here. I wonder if we're talking about the same modification to the trackwork. As I remember, all the 60s and into the very early 70s, the crossover UF>US and DS>DF was south of Bromham Rd Bridge, perhaps a chain or two north of Bedford North Box. The semaphores controlling the approach from the north had a fixed distant for any up express stopping at Bedford MR - the equivalent in those days for an approach control release with colour lights.
This crossover definitely came out before I left Derby in late 1974, well before the changes for the rebuilding of Bedford station. I remember it was moved north of Bromham Rd bridge - and was replaced by an approach controlled (I think single lead ladder, but not sure on that) because I can remember looking out on up semi-fasts with Cl 45s slowing to a trundle for 200-250 meters with the UF signal at red until about 40m from the signal, when he would get a single yellow (I think) and the feather indicating the switch to the slow line.
As I remember it, this new crossover was around MP 50 1/4. I think it was a 20 mph restriction, so with a Cl 45 and 8-9 Mk1s on the drivers usually had to apply a bit of power to get the train into the station after the crossover - making the new layout a right waste of time and power compared to the old one - although I assume it was safer and cheaper to install and maintain.
I took note of this because I wondered if, after it had been installed for a month or two (and crews were familiar with it) they would relax the conditions in some way to reduce the time lost. And IMX, they never did.
This had to have happened before 1974, because I never went on the up to London again until 1990. I would have guessed probably 1973. If you say it didn't happen until 1978, I'd better call the men in white coats asap.
Could it be there was another modification introduced in 1978 that you are thinking of?
I confess that though I must have used the new crossing in the down direction, I can't remember that. Probably because the additional time lost by the new arrangement was not so damaging as on the up.
I can't argue with your dates for the removal of the crossover to the south - as I remember it, the old crossover was north of the bridges over the Great Ouse, and must have been on the curve of the avoiding fasts - meaning a non-standard design that was surely expensive to build and maintain. Hence the move 1/4 m or more to the south, on straight track, at Kempston Rd. But of course, this change also added to the 'trundle time' for any Class 1 train stopping at Bedford, both on the up and down. On up stopping trains, they definitely had to give a burst of power between the crossover and Bedford Sth Box.
