Crossrail

Crossrail is a railway line under development in the London area. It goes to the counties of Berkshire, Buckinghamshire and Essex, England. 
The central section between in central London and in the south-east, is open. It will later join with two existing routes operated by TfL Rail to become the Elizabeth Line, named after Queen Elizabeth II. 
TfL Rail's Shenfield branch is connected to Paddington and the branches west of Paddington are connected to Crossrail's central core. This completes a new east–west route across London. It will provide a new high-frequency commuter and suburban passenger service.
Basically, it is an additional NE to SW connection, ending up in Heathrow. Previously, it has been difficult to get to and fro between N.E. London and Heathrow, so this will help that journey.