Those are fibre optic cables on 'remake' frames. It is not just about providing slack in case a repair is needed. In order to achieve good joints in an optical cable the fibre needs to ideally be the same diameter on both sides of the splice. However, the thickness of a fibre does vary in the length and of course with a multicore the thicknesses of individual cores will not be guaranteed to be the same at any given point. Within the joint housing, allowance is made for slack to be coiled either side of each individual splice, each splice and coiled slack held in its own enclosure called a tray. Thus there is some flexibility in where each splice is made within a cable and the 'remake' frame is a means of neatly tidying up the slack.
In the event of a copper cable being damaged by train derailment, blown out traction section switch, exploded 11kV or 22kV main or some equally destructive disaster, standard practice would be to run in a new length of cable and 'closure' joints to bypass the area of damage. I expect the same would apply to the fibre optic cables in most cases but it is 10 years since I retired and I have no idea how the technology and working practices have moved on in that time.