I worked for 27 years with London Underground before retiring to Finland in 2001.
About five years ago, VR (Finish Railways) introduced double deck coaches onto some of its inter-city routes. Over the bogies there is a mezzanine coach floor which provides access to the adjacent coaches.
Between the bogies, the floor is lowered to 300mm above rail level and there is a second deck. Both decks have about 2.2m headroom.
The double deck coaches have been so successful that VR intends to replace all single deck intercity and express coaches over the next few years.
However, the height of the British kinematic envelope is about 4100mm. The coach itself is around 3.9m high with the normal coach floor 1100mm above rail level. The platform is 925mm above rail level.
Allowing for a floor level of about 300mm above the rail and roof construction, gives less than 3.5m for two decks - clearly this is not practical.
Double deck trains do have a place in countries where the structure gauge is big enough to accommodate them but they are not a universal solution and there is unlikely to be a financial case for their introduction in Britain.
Chris O'Hanlon, 12100 Oitti, Finland, chris.ohanlon1@kolumbus. fi