Valley Line West includes an expansion to the existing Gerry Wright facility, which was built to accommodate expansion. Gerry Wright will be the main OMF for the whole Valley Line, with the facility out at Lewis Farms being for light maintenance and storage. I think the idea behind the phasing was that when the VLSE was awarded in 2016, there was no timeline or construction budget for VLW, so they chose to only build enough of an OMF to support the VLSE initially. here's
Marigold's (rather sparse) page on that part of the project.