(rail transport,colloquial) the area between the closest rails of two parallel standard gauge railway lines, regardless of the actual distance.
1882, George P. Neele, Atlantic and American Notes, M'Corquodale & co., limited, Page 54
Cattle are of course liable to stray on the line at these level crossings, but to prevent this, barriers are placed on each side of the crossing, and a deep trench is made in the four-foot and six-foot spaces, [...]
1981, Ludovic Henry, Coverly Kennedy, A Book of Raliway Journeys, Fontana, Page 21
He fell wildly, his head struck the carriage footboard with tremdous force, and he bounded into the six-foot, where he rolled over and over [...]
2007 September 5, Rail Accident Investigation Branch, Rail Accident Report 33/2007: Fatal collision between a Super Voyager train and a car on the line at Copmanthorpe 25 September 2006, Rail Accident Investigation Branch, Deptarment for Transport, Page 20
As a result, all three wheelsets derails to the six foot side.