Coaches were built there for Royal use from the 1840s but the 1869 saloons built for Queen Victoria set new standards. Carriages were limited in length because of the need to negotiate curves on the track safely, but the royal entourage demanded more space than could be satisfactorily squeezed into a single carriage. Victoria, however, was very nervous about stepping from one carriage to the next, so the Wolverton engineers came up with the idea of a bellows unit that could connect two carriages without exposing passengers to the elements. Before long this feature became standard on all trains. I have discussed some of this here.
Here are some photos: