Why does the UK not use double decker trains and trams?

The biggest impediment to double deck trains in the UK is not, as some believe, the height of the loading gauge, but the platform designs. For double deck rolling stock to be viable, the floor height on the lower level needs to be low floor, close to the rail head height. With UK platform heights that would put it below platform height. While that isn't in itself a problem, as other places with high platforms use double deck stock with the lower deck below the platform height, the problem with the UK is that the horizontal distance from the track to the platform edge is smaller than in a lot of other countries. That means to fit a lower deck below platform height within the loading gauge, the lower deck would need to be extremely narrow, perhaps 1+1 seating at most. That width does not allow for enough added capacity to make it a worthwhile improvement over high density single deck stock. The majority of UK first generation tramways ran with double deck cars, but the transition from fixed frame trams to articulated trams, and in particular low floor designs, makes it impractical to produce a double decker design. There are few if any double decker designs with articulation that allow open passenger circulation on both floors, generally they have either mid height gangways or gangways only at one level (eg TGV duplex with gangways at the upstairs level). The general desire for trams to be open with large circulating spaces for high passenger volumes does not work well with this design paradigm, and single level articulated tram cars have quite high capacity anyway. Double deck vehicles, trains and buses, suffer from a dwell time penalty as more passengers try to use relatively speaking fewer doors, and for tramway operations, it is generally the case that high frequency high capacity single deck articulated cars provide the best solution for a tramway service pattern. If a modern tramway system was built with platforms too short, the capacity problem should be solved by providing longer platforms and procuring longer vehicles.