Part of the reason railfreight isn't economical in the UK is the fact that distances from the sea just aren't long enough. In the UK, you are never more than 70 miles from the sea.
If it wasnt economical, why do FOCs actually exist? FOCs are private sector companies. And I rather think that freight trains travel more than 70 miles on an average journey ( Southampton and Felixstowe to Trafford park spring to mind.
As a few have stated , railfreight is high bulk movements.....clearly a train with one driver/locomotive with 70 containers in tow has a significant cost advantage on a journey between Southampton and Trafford Park than 70 HGVs with 70 drivers....