The mention of the Rotherhithe tunnel leads me, and most Londoners, to think of the road tunnel which was opened in 1908.
The Brunels were involved with the Thames foot tunnel between Rotherhithe and Wapping, which was bought in 1865 by the East London Railway Company and now carries the East London tube line under the Thames.
Brunel may well have referred to the 4ft 8in railway as "narrow gauge". But the 4ft 8in gauge is more correctly known globally as "standard gauge".
"Narrow gauge" is used for track narrower than standard where shorter wheelbase rolling stock and tighter radius bends can be used.
Upper Elmers End Road