The risk analysis is fundamentally flawed in my opinion.
Third rail lines are in more populous areas, often in high crime areas where trespassing would be expected, unlike the long, rural ac intercity lines.
Secondly, the railway is intrinsically hazardous because of its high energy levels; mostly these are kinetic but some are electrical. The video on-line of an Indian express hitting a sheep (don't watch it!) shows how much greater danger kinetic or movement energy is than electrical energy, as many electrocution incidents are injurious rather than fatal. Indeed the argument can be made that 750Vdc causes fewer fatalities per contact than 25kVac.
Thirdly, electrical safety regulation previously specifically exempted the railway from general H&S rules. The current inclusion is particularly reprehensible in that it simply adopts European standards for no UK scientific reason (because BR/NR couldn't show where its previous standards came from.)
That is not to say that Headbolt Lane's batteries are a bad idea. A successful trial will open the way to Merseyrail (and others) being able to operate out of area along lines that would not see electrification however slack the rules. What is disgraceful is the concrete block and the misaligned, differing levels - there are actually two terminal stations; HBL HL and HBL LL, that are unlikely ever to be joined economically.
The optimum approach for dc lines in my view would be to convert Merseyrail to ac, with batteries in confined spots such as the tunnels and to allow completion of the few internal SR dc electrifications, with conversion of the SW main line beyond Pirbright Jn to ac. There should also be improved fencing/protection etc. This would be a better, safer railway but it will not come to pass.
WAO