Photos:
Tony Lyster - Up Yard locomotive Ashing Out Pit

There being no engine shed or watering facilities at Quainton this pit was probably constructed during wartime at the Aylesbury end of the Up Yard near the yard throat. In recent years, with the corrosive effect of locomotive ash, the sides eroded and crumbled so it has now been rebuilt (2005) on a slightly different alignment, but to the same length.

References:
The Up Yard Track Re-laying Caper - Quainton News No. 68 - Winter 1990 / 91