If you do a web search for CBOA (Commercial Boat Operators' Association) or look at Waterways World, which has a section on carrying news every month you'll see what is happening - it's only really on large rivers, especially in the north-east, that barges can carry enough (200+ tons) to outweigh the speed differential. Even in the canal age, the canals built mainly to carry agricultural loads (as opposed to coal and manufactures) weren't profitable.
Sean
