New platform at Denham
Reader THC contributes this photo of the new platform being built at Denham station* on the Chiltern Main Line:
They've built this because the existing westbound platform (left) is slipping down the embankment. It's accessed by a new footbridge which includes lifts, rather than the subway to the existing platform.
The station was originally built with space for four tracks so that express trains could run through the middle, but these have long since been removed, leaving plenty of space between the two remaining tracks for the new platform. The station's Wikipedia article has a good "before" picture.
* Outside Greater London but inside the M25, so it's just about on-topic