London Transport have some interesting contributions here. The 1987 tube stock for the Jubilee Line with its single doors was never likely to be suitable. I don't have precise dimensions, but they certainly look to be narrower than the single doors on the District Line's D stock. Some of the 1987 Mark 2 trains spent more time rotting "in store" than they did in service.
The uncompleted parts of the Northern Line extensions from Finsbury Park to Alexandra Palace, from Mill Hill East to Edgware, and from Edgware to Bushey Heath. Work stopped early in the Second World War. Afterwards, Green belt policies meant that the additional housing that would have produced passengers for the Bushey Heath extension could not be built. The UK's serious financial difficulties led to the cancellation of the other schemes. But large amounts of money (for the times) had been spent that in the end was wasted.
The electrification of the Central Line from Epping to Ongar in 1957. It probably made sense (a) to complete the plans formulated before WW2 and thus replace a push-pull steam shuttle, and (b) to do it using materials salvaged from the Finsbury Park to Highgate section, but the long-term financial viability of the line was nil.
The section of Jubilee Line from Green Park to Charing Cross, and the tunnels beyond Charing Cross nearly to Aldwych, also deserve a mention, but it wasn't LT's decision to change the plans and extend the Jubilee Line on a different alignment.