P65 on. 

This is from Brunel's July 1838 report to the board. 

To my mind the report contains some special pleading and rather mixed logic! 

At this stage Brunel is envisaging carriage bodies set between large diameter wheels, and there is no thought of mixed gauges. 

He notes that larger track gauge does not necessarily mean a larger loading gauge. 

It appears that the concepts of loading gauge and structure gauge are still somewhat loose at this time, although Brunel is writing for a non technical audience and doubtless must keep to plain language and simplify. 

He notes that the Liverpool and Manchester was constructed with 9'10" between verticals -, eg structure gauge, and this proved too small. The London and Birmingham was constructed with 11ft between verticals, and this is considered too small by some (wonder if by this he means himself!) 

He proposes to build the GWR to 13ft between verticals to permit 12ft loads, mentioning hay and other light weight traffic. 

He also talks about tunnels, clearly twin track. The London and Birmingham has 24ft tunnels, and he purposes to build the GWR to 30ft tunnels, "more with a view of diminishing the objections to tunnels and maintaining the same minimum space which hereafter may form a limit to the size and form of every thing carried on the railway, than from such a size being absolutely necessary." 

And from P211 in the section on the Gauge war, it mentions that Wickwar and Fishponds tunnels on the Bristol and Gloucester, which was planned as narrow gauge but constructed as Broad gauge, were 26ft.

To put this in perspective, here are leading dimensions from when structure gauges were more firmly defined. 

By 1872 the GWR broad gauge structure gauge was 14ft9 wide at height, 14ft3 wide above the platform, and 11ft 2.5in wide at platform height. 

In 1908 the Narrow gauge GWR structure gauge was 14ft 2.5 in wide, and 25 ft. 5in for double track. 

By 1950 the official requirements from the Ministry of Transport Board of Trade were 25ft6 minimum, 26ft6 desirable for double track, and14ft4 minimum, 15ft4 desirable for single.