In the days of steam there would be spare carriages at most medium to large stations and a carriage or two
would be added if required by the station pilot. (A locomotive set aside for such work).
Most local platforms can handle four carriages and the current design of concrete uprights bridged with concrete means that extending is simple enough. Some platforms are still wood.
Double decker are very unlikely due to this country being the first with railways and Stephenson loading gauge and rail gauge being adopted. They should have listened to Brunel.
The fact remains that in my travels I see hundreds of miles of route where quadrupling of track could be done without much disruption to current services. In some case the abandoned track is still there.
HS2 is folly. Route pricing is more influential than saving 20 minutes unless your customers are paying.
