London Reconnections reports that:
Train testing on the East London Line Extension (Phase 2) between Surrey Quays and Clapham Junction has now begun. The route was fully electrified at the weekend, and the first 378 ran on Sunday. Driver training will begin shortly, and further testing will now take place, ahead of the extension’s official opening in December.
The impact that the ELL extension will have on Brockley travelers is uncertain. It will mean more trains through the the core route, from Dalston to Surrey Quays, but it will also mean more passengers at Canada Water. Although it won't provide any new direct destinations, it should make travelling to South West London a little more convenient.