I would love to see Nottingham-Glasgow figures? Is there any meaningful connection between them?
I would think it was Nottingham (and Derby) to Stoke, Liverpool, Manchester, Chester/NW, Preston/Blackpool even. Even though there is a single direct service, it is slow and unreliable - and the reversal at Sheffield is frustrating. Stoke specifically offers multiple fast Manchester services on longer, better trains - as does Crewe for Liverpool and Lancs/Lakes.
No wonder it is being used more - I agree, it's a very underused corridor and more regional express services would be an interesting thing to explore.