Thank you for letting me join. I hope my info will be useful.

I am new to this and may have just issued a reply in the wrong way! Anyway, here it is. Sorry if it appears multiple times.

Only just spotted this and I imagine you have all the info you need, but I?ll still add my contribution. The track was indeed slewed across on 01 February 1976; I was there in person to watch it. Here is some background information.

The work was undertaken in order to iron out a tight reverse curve on the Down Main line at the west end of the station. This reverse curve caused a speed limit of 90mph. With the introduction of the new HST^▸ (High Speed Train (Inter City class 43 125 units)), 125mph was required and so the solution was to slide the track across. This removed the problem and allowed the full line speed to be realised.

This was the only reason for the work; the former platform 1 having already been extended through the site of the old West signal box in order to accommodate 12 coach trains. Incidentally, the platform extension was achieved by reusing prefabricated concrete platform sections recovered from Box Mill Lane Halt. These were removed from Chippenham within weeks of the track slew taking place.

The line through Chippenham (Wootton Basset to Box) was resignalled for the coming of the HST; signals re-spaced for the higher speeds, additional signals provided to shorten long block sections, and bi-directional signalling provided for additional flexibility. Some of this was installed prior to the track slew while the remainder was completed a while afterwards.

Because Chippenham had already been significantly rationalised before anyone thought about the track slew, the track slew itself didn?t require any signalling alterations except for three relatively simple adjustments. The bay platforms and associated connections had already been removed a few years before, leaving only a simple ground frame controlled connection to the down side yard. When the track was slewed, this connection was abolished and so the ground frame circuitry had to be removed from the signalling system.

The next signalling adjustment was to a signal at the west end of platform 1. This colourlight repeater signal (SN72R) had been provided only a couple of years (or so) before, in readiness for the overall signalling upgrade. When the track was slewed, this signal was moved sideways to follow the track. This signal was later to become a full stop signal and renumbered SN70 as part of the completion of the signalling work.

The third signalling alteration involved the relocation of the barrow crossing and the associated white light indicators. As the lights were driven by the signalling system, moving them from the east end of platform 1 to the west end caused a change to the track circuit (train detection) sections that controlled them.

Much of the platform face wall of the old Weymouth bay (which became the new Down platform) had to be rebuilt as a new concrete block wall, located further out to meet the new track alignment. This made the island platform noticeably wider at the west end. Only a short section of original red brick platform wall remains visible, located under the platform canopy. The east end, as already noted by others, was widened due to the infilling of the Calne bay and the removal of the old up-side buildings.

Finally, the two footbridges were raised by a couple of feet in order to increase headroom, the new track was slightly higher, greater aerodynamic clearance was required, and unobstructed sighting of the new signal at the west end was necessary.

Overall, it seemed to be a lot of work just to raise the speed limit but in those days two out of three trains ran through Chippenham non-stop and so it made complete sense. Also, dozens of these speed raising projects were undertaken between Paddington and Bristol (including the Badminton line), and lots of seemingly minor speed improvements added-up to a considerable time saving overall.