The speedo design is very poor, measuring voltage of an axle mounted generator. The meter is of the type in which the response diminishes as the voltage increases, so that the needle moves a lot at low speeds, but the really critical bit around 15mph plus, there's very little extra movement. There ain't much margin in the traction motor design to overspeed and burst the windings above this, as several guys have found running light on the main line.
The LMS, who pioneered the design with English Electric pre WW2, had two parallel versions, one with a single motor and jack shaft drive needing a longer wheelbase, and the one we know with motors on the outer axles giving a shorter wheelbase. This was dearer, but the BR powers that be went for the short wheelbase for tighter siding curvature for a standard.