Actually, it was the other way around - the line had originally been built as a relatively minor branch line from High Wycombe to Thame, and so it traversed the Chiltern escarpment on a steep 1:88 gradient to avoid either extensive earthworks or a tunnel.
When the line was repurposed as the Great Central Main Line at the turn of the 20th Century, this gradient was deemed too steep for the heavy freight traffic the line would see, and so instead of doubling the line along the existing steep bank, a second, shallower alignment was built at the line's new ruling gradient of 1:176. This necessitated the earthworks and (short) tunnel the Wycombe Railway had avoided originally.
Thus the 1:88 bank is now only used by down trains and there is no signalled move in the up direction. With the tracks running on separate formations for approximately 2 miles (if defined by the points between which there is no ballast in the "six foot") this is possibly a decent contender.
