Railway limits come from infrastructure, not a horse-width myth

Do British railway limits ultimately come from the width of two horses, and why are double-deck trains uncommon?

No; the horse-width story is a myth, while inherited tunnel heights, bridges, and platform lengths constrain the loading gauge available to modern British trains.

The popular claim that standard railway gauge derives from the width of two horses is rejected as a myth. The standard changed during railway development, while Isambard Kingdom Brunel's broader gauge is presented as offering better long-term potential for speed and capacity. For modern British trains, tunnel and bridge height often matter more than track width alone. Those loading-gauge and platform-length limits make double-deck trains difficult to introduce on much of the network. Whether planned HS2 infrastructure will accommodate double-deck trains remains uncertain, and a proposed central Scotland extension would still inherit bridges on the older network.