The WCML has to accommodate very long distance express trains, regional trains, suburban/commuter trains, and freight. I think you can read section 2 of [1], a

The WCML has to accommodate very long distance express trains, regional trains, suburban/commuter trains, and freight. I think you can read section 2 of [1], and the start of section 3. With enough trains, you can still fill up a four-track railway. [1] https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/480647/annex-demand-and-capacity-pressures.pdf https://assets.publishing.service.gov.uk/government/uploads/...