//(services run on parallel line almost empty much of the day)//
not so Davebro. on the west coast line, trains are full and standing all day, every day. there are now no more train paths available south of Stafford, and certainly no wiggle room to take any more freight services. the last upgrade of the west coast route was meant to be future proofed but all the spare capacity has gone - this is notwithstanding the railway equivalent of "smart motorways" (lengthening the pendolinos and allowing the slow trains to run at 110mph), initiatives that produced capacity that is itself now all gone.
the suggestion is another upgrade. but, er, how? in any case if it proved to be doable, (for instance lengthening the platforms at Liverpool Lime Street would require major engineering because of the tunnels at the platform ends) it'll leave rail travellers with 10-20 years of disruption, with weekend shutdowns and regular 4-6 week blockades (Bertie the bus - deep joy).
the only other option is allow 80T trucks on the road. the economics of railfreight are on such a knife edge that such a move would kill it overnight; that would provide some capacity on the network but without freight trains, growth must necessarily then fall back on the roads. would that be an environmental price worth paying?