Even zooming in, I can't see how they achieved that. I'm presuming OOC/Willesden are involved somehow, but maybe there is another way... www.openrailwaymap.org//mobile.php?...

OpenRailwayMap

OpenRailwayMap - An OpenStreetMap-based project for creating a map of the world's railway infrastructure.

Acton Main Line station then off the mainline railway and onto the freight line. Then a little bit of the London Overground, and back onto freight lines up to Willesden Junction station, past the London Overground trains and onto where the train could join the mainline into Euston.