I totally agree that there was plenty of room for an extra track, not only on the down side as you point out but also on the up side, which was built as a siding on which the redundant Starlight Special coaches were stored in the early sixties.
In fact, it looks to me as if the GCR made provision for through of by-pass lines in both directions, so that --- if traffic levels increased --- expresses could rush through Brackley uninhibited by the 'turnback' trains that terminated at Brackley. From an operational perspective, this all made perfect sense.
On the other hand, if the GCR had even the slightest intention to make Brackley a junction station, it would have made provision for a means for passengers to reach this notional extra platform. As things were, there was not any room to spare at upper level to create a landing for an additional staircase. The circulating area was quite modest, with the booking office and the SM's office on your right and the lockup 'cage' for parcels and bicycles on the left. Straight ahead was the footbridge.
To create a 'Northampton platform' would have involved major reconstruction to the station building, whilst at track level the GCR would have had to make vast excavations for the diverging tracks to Northampton in the cutting north of the station.
And to what avail? Northampton was already served by two LNW stations and one Midland Railway, providing a direct service to London with frequent trains. The GCR could not realistically hope to tap off any of this traffic along a longer, roundabout route to London via Brackley, involving a change of trains and a wait at a station that had neither a refreshment room or a bookstall. Local traffic between Brackley and Northampton would not be great; most locals looked to Buckingham and Banbury for doing shopping that Brackley shops could not handle (the bottom station in Brackley offered a direct service to those two towns).
By and large, the GCR was interested in the bigger and more lucrative picture of' long-distance passenger and freight traffic in the 1890/1900 era. Why would be interested in a tuppenny-hapenny feeder service from Northampton, especially one constructed through quite heavy terrain?
I am not trying to be a contrarian or trying to stop discussion of might-have-beens. But I feel I must emphasize that the Northampton platform story is a myth (the Wikipedia article https://en.wikipedia.org/wiki/Brackley_Central_railway_station also points out that the so-called platform t it appears to be located on the "wrong side" of the station, since Northampton-bound trains would have had to cross the main running lines in order to access the branch). It was just a schoolboy joke made to impress his friends!
Jim Goodman said: