Short answers, pulled from the story.
The Khyber Pass is in the Khyber District of the Pakistani province of Khyber Pakhtunkhwa, along the international border with Afghanistan. It connects the town of Landi Kotal to the Valley of Peshawar at Jamrud, traversing part of the White Mountains. The summit lies 5 km inside Pakistan at an elevation of 3,373 feet.
The pass proper runs 25 miles from Jamrud to Landi Kotal within Pakistan. An additional 8 miles extend westward into Afghanistan toward the town of Loya Daka.
The Khyber Pass was used by Achaemenid and Sassanid forces, Mongol commanders including Duwa and Qutlugh Khwaja, and Islamic rulers such as Mahmud Ghaznavi, Timur, Babur, and Nader Shah. The Saka, Yuezhi, and White Huns also passed through it as nomadic invaders from Central Asia.
The British government of India constructed the Khyber Pass Railway after World War I, running from Jamrud near Peshawar to the Afghan border near Landi Kotal. It opened in 1925.
Almost 80 percent of the NATO and US supplies transported by road into Afghanistan moved through the Khyber Pass after the invasion began in 2001. Pakistan closed the pass to NATO supply trucks in 2010 after a NATO helicopter attack killed three Pakistani soldiers, forcing the alliance to develop alternate routes through Central Asia.
In Cockney rhyming slang, "Khyber Pass" means "arse". This usage was made explicit in the 1968 British comedy film Carry On Up the Khyber.