You still need a powerful and heavy loco for the limited freight that we do have. Last year 084 struggled with an IWT liner out of Northwall, it made a few attempts and 071 was even used as a banking loco to get the train to North Strand. 084 finally failed outside Heuston and had to be replaced. Late last year 201s were struggling hauling liners out of Northwall too with poor rail conditions. These are 600-700 ton freight trains and 110ton 3,200hp locos are having a hard time.
Lighter bo-bo locos would be no good for these jobs. Also once you go bo-bo your axle load goes up, just look at the new Class 68s in the UK. We could do with locos like that here but they would need to be co-co configuration but the 68 design does not allow for it.