Oh you are a cynical lot!
This is exactly what they did at Victoria Station ten years ago and the resultant development is a huge success!
They created two stories of prime central London shops, resteraunts and offices over the splatforms - literally out of thin air!
With regards to noise disturbing residents I assure you that even a blind mouse sitting upstairs wouldn't be able to hear the 17.15 Gatwick Express trundling out!
Go and have a look if you don't beleive me - admitedly there is more traffic at CJuncton but modern soundproofing techniques can soon alleviate it.
It leads me to think that the acres of railway lines criss-crossing London could easily be built over - hiding noisy tracks and ugly track furniture whilst providing building space for literally millions of homes in prime locations all over London - Imagine the stretch from Waterloo to Wimbledon alone!
Reminds me of a very old song about "Trains coming through my house" complete with steam and smoke rising though the floorboards!