4 foot 8 and a half inches apart.
Used to be, now they're four foot eight and five eighths.
Shortly before denationalisation, I spent a while working at British Rail. The people I was with were immersed in railways. One explained to me how it had been found that trains self-centred themselves better with old, barrelled wheels than with new flat ones, and went round bends better with an extra 1/8th inch on the gauge. So over several years, during maintenance, the tracks were relaid with the bigger gap.
I was mostly at Darlington Low level, one of the world's most historic railway sites, though dwarfed by Darlington High Level, which is a stunningly huge station for a small Northern town.
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