Well, OK not technically a day, but practically speaking it bought me around 7 extra hours in Edinburgh, which felt like an extra day in comparison to having to spend 8 hours+ on the return train journey.

There are many other practical factors about my specific journey your comparison doesn't include, such as the fact that I don't actually live in Oxford, so I need to get the bus to Oxford station (in which time I can get 90% of the way to Birmingham Airport by car), the times of day I was travelling, connection times, and travel time at the other end of the train journey. (And if you factored in Oxford's marvelous road works, that's probably another 4 hours saved right there ;-)

As for a miserable journey... last time I went to Edinburgh by train it was so overcrowded they simply cancelled all the seat reservations altogether, because people couldn't get to their seats. So I stood/squatted/sat on luggage in a crowded hot train for hours. Not fun at all. I knew would be busy. That's why I bought a "reservation"!

The plane journey was a doddle by comparison.