On October 27th during the Second Battle of Passchendale, Harry and his Observer spotted three large freight trains under the cover of bad weather and called in an artillery strike which hit all three trains. One of the trains was probably carrying munitions as it blew up, completely destroying all 15 carriages.
After three long months Harry finally saw the capture of Passchendale on the 6th of November 1917. He was looking down upon five miles of ground that cost 325,000 lives. He had been flying for six months over the Somme where the average life expectancy for a pilot was just 21 days. He had flown 252.40hrs over the Somme. He was finally given leave and returned to the UK.
See also HD Williams Memoires pages 24-25