Indeed, there is a wide choice of "official" values for track interval (the "six foot") or spacing between centres:
Even the note "Guidance on Gauging" (GE/GN 8573) gives a number of values, but none that is the definitive answer:
1) ^8.1 A possible structure gauge for Britain^ in which the track interval is 1970 mm (track centres at 3405 mm). "The structure gauge model is based upon the traditional
BR▸ structure gauge defined in BR Handbook 4 and further described in withdrawn
GC» /RT5204."
2) ^8.4 UIC compliant structure gauges^ for straight and two curve radii, giving track centres of 3597 mm, 3858 mm (on 250 m curve), and 4092 mm (on 150 m curve). "The [^] structure gauges have been developed from UIC leaflet 505-4 [^]."
3) For high-speed lines, under the heading ^Part 10: Requirements Relating to Gauging in the High-Speed TSIs^, it says:
Distance between track centres
The minimum nominal distance between track centres on upgraded lines in Great Britain shall be 3165 mm.
10.2.2.11 Part G of GC/RT5212 sets out requirements for new, altered and temporary infrastructure, including track intervals. These requirements could require a greater distance between track centres than the minimum set out in the TSI.
10.2.2.12 The accepted distance between track centres in Britain is 3400 mm for straight track.
The main relevant parts of GC/RT5212 are:
G1 Minimum upper sector clearances for new infrastructure
G1.1 Requirement for normal clearances
New infrastructure (as defined in section B3) shall be designed to provide at least normal clearances (as defined in section C2) to all types of rolling stock that use the route or can reasonably be foreseen to use the route.
The required clearances shall be calculated in accordance with Part D of this document.
G1.2 Additional clearances at vehicle window level
The following clearances shall be provided at the level of opening vehicle windows (normally between 2000 mm and 3000 mm above the plane of the rails):
a) 450 mm where vehicles operate with opening windows allowing passengers to lean out
b) 250 mm where vehicles operate with opening windows for the use of train crew.
GM/RT2456 requires passenger door windows that are capable of being opened (drop lights) to be locked so that they can only be opened manually by train crew. This requirement is to be implemented by 31 December 2005 for all vehicles that are intended to remain in passenger use after 31 December 2007.
G3 Track intervals for new infrastructure
For each section of track the infrastructure controller shall determine track intervals to be used for new infrastructure, taking into account:
a) the requirement to provide normal clearances, as set out in section G1.1
b) where necessary, the requirement to provide additional clearances for opening windows, as set out in section G1.2
However, it should be noted that this document tells you how to set your own track interval, rather than specifying a value for you, and covers many other relevant factors.
Note that a gauge of 4^ converts to 1435.1 mm. This is rounded to 1435 mm in the definitive value in GC/RT5021 ^2.9.1.1. Track interval is generally measured between the gauge faces (i.e. it includes one rail of each track).
So there you have it - if there is an answer it's 3.4 m between centres on straight track, a bit more on sharp curves. The lines through Reading are, as far as I can measure them, at 3.4 m. That appears to have been set to allow for you sticking your head out of a window; if that's no longer a possibility then maybe the spacing could come down in future.
