There's just one action GWR▸ should take.
Ditch Capita.
Possibly, but you need to understand what specification they were given to work to first, and consider the alternatives - are GWR willing to (re) invest in an entire Customer Service operation?
For example, if Capita were told that (for example) 90% of replies were expected within 28 days, they may well be working within their contractual tolerances.
In this sort of scenario, they may well have been given standard paras to use by GWR for the majority of their answers.
If Capita aren't doing anything "wrong" contractually, it would probably cost GWR a fortune to extract themselves from the arrangement, or insert variations to allow for stricter KPIs.
If they've been "crap" from Day 1, and are missing all their targets after this length of time, then you have to look at the way they are/the contract is being managed by GWR as well as their performance. In my experience it is often the way the contract has been let and is being managed as much as the performance of the outsourced agency, and that can only really be managed objectively, by comparing their performance against what is set out in the contract.
Capita aren't universally bad, although they do get an awful lot of bad Press.
Sometimes it's as much about looking at the discipline of the parents as well as the badly behaved child?