Mikkel,
I think it is difficult to say if it resulted in decongestion as London was, and still is, an expanding city. People obviously preferred the Underground as even though it was full of smoke and steam very quickly it was overcrowded. A much expanded system later, it is still overcrowded.
In answer to today's question, what is known is that when we have a Tube or Train strike the amount of cars coming into London dramatically increases. What is also known is that the Congestion Charge reduced traffic into London.
Hope this helps and does not hijack your thread. The reverse is that the motor car stopped this country/ the western world, (?), from being drowned in horse poo and starving to death as we tried to feed the horses.