All the line in this area appears to have originally been a 1' 6" gauge, including a 1½ mile spur line from Chattenden to Hoo. However the spur line which this record concerns appears not to have been converted along with the other lines in 1875, to a 2' 6" gauge.

From the source: “Chattenden Barracks (just behind the Railway Depot) was then the centre of the School of Military Engineering. By 1875 work had progressed so well that there were some four miles of track from Pontoon Hard, via Tank Field and Church Crossing to the Railway Depot with a spur line going from the crossing towards Lower Upnor, as well as a one and a half mile line from Chattenden to Hoo Creek.”
(1)

<1> D. Yeatman, 1966, Lodge Hill & Upnor Railway (Article in serial). SKE16256.