Horses used to pass through it on their way between their stables and Ascot Racecourse. It had a specially designed floor to be kind to horses' feet.
The underpass survived for decades, hidden behind tall grass and weeds, just a few metres from busy Botany Road.
All that remains today (9.8.17) is a short section of brick wall, with a few piers. If you want pix, you'd better hurry.