I looked at the cost difference between getting from Glasgow and London last year. It worked out slightly cheaper to fly, but the time spent in getting to and from the airports made it easier to get the train.
Certainly for various destinations the longer 'travel tine' of a train is readily offset by the long wait at baggage check, security check, cavity-searches, etc. And you can walk around on the train rather than folding yourself into a seat that would be too small for Dr. Miguelito Loveless.