Lots of publicity is currently appearing in all Overground stations, including a nice map showing you what work is taking place at Overground stations to extend the length of the platforms to accommodate five-car trains.
Overground extension map
The new longer trains should be ready to roll out and be in operation from December of this year, and you can see work taking place all over the Overground network to lengthen platforms, as can be seen here at West Brompton.
West Brompton Extension