Maryport doesn’t become a single line either, although trains in both directions do use the same platform - the Carlisle->Barrow line continues through on the east side of signal box, while the line in the Barrow->Carlisle direction continues through to the west of the signal box. Trains in either direction divert off to the single platform though, so not sure if this satisfies OPs requirements? Not sure if it’s supposed to happen, but you’ll quite often have a Barrow bound train have to wait for a Carlisle one to clear the station.