Sentence:  In the Gladstone photo, the sitters look too small for their conveyances, and this is because they were sitting in extra-wide - that is, <WSD>broad-gauge</WSD> - wagons of the Great Western, which would run along the Met on <WSD>broad-gauge</WSD> tracks (7 foot 2 inches between the rails), together with trains using standard-gauge tracks of 4 foot 8½. In the Met's own language, the line ran on 'the mixed-gauge principle', which makes the arrangement sound almost sensible. That broad gauge did not last long, and all Underground trains today run on the standard gauge. 
Meaning of the word marked with <WSD> tags:  Having a wide gauge.