Richard J. wrote:

But until you do, you can't use the new stock! To accommodate the two
heights at a single platform, the platform level needs to be a
compromise. If it's level with either stock, the other stock would be
unreasonably low or high. Anyway, where does it say that S stock
floors will be at tube stock height? I don't believe that's true.

Neither do I. How would that work on the Metropolitan, where all platforms
are "normal" height and platforms from Harrow northwards are shared with
standard gauge overground stock (Chiltern)? Utter rubbish IMO.