In the firebox, look you? Didn't his scales get filthy?

While I remember, given fairly hilly / mountainous terrain and dragon-sized bodies - and a dragon liking for travelling in open-topped wagons so that they can stretch their wings and fly if they feel like it - would the dragon railways be better off going for something like standard gauge, broad gauge, or narrow-gauge? Assuming I've spelled gauge correctly, which I doubt (but this isn't the PC with the UK spell-checker).