GWR would likely lose the case if they tried to defend in court. The breaches are quoted as
ROGS 2006 Regulation 19 (1) and
Health and Safety at Work Act 1974 Section 3(1), neither of these require an accident to be caused. The ROGS one is to have an appropriate risk assessment and implement the measures from it. The other is to as far as reasonably practicable not expose non-employees to health and safety risks.
The two relevant actions from the risk assessment were extra staff training and revisions to signage around the doors. Neither of these would be particularly difficult or expensive (so are reasonably practicable) - the target date for all four actions (including two unrelated) was May 2018. The staff tasked with those actions left, the action tracking system did not notify anyone these were overdue or assigned to people who had left. No one had realised in six months after that date before this incident that these items weren't done or being actively progressed. That's slam dunk on the ROGS one, they didn't implement the measures and it was organisational failings below what could be reasonably expected to blame. Similarly the other one the only defence would be the actions wouldn't reduce the risk exposure, but that is undermined by GWR recognising those as appropriate actions in its risk assessment.
Many of the issues uncovered in the investigation would have been present for other safety risks.
The circumstances of this case make any conviction on those charges unlikely. While clearer signage could reduce the number of people putting themselves at risk, for any manslaughter charge would need to prove the actions to alter the behaviour of
this group. While it doesn't need to be definite, it does needs to be more than a possibility and reporting suggests the group knew it was a risky activity so would be a tricky bar to clear.
The significant contribution issue wouldn't apply to the windows being there. However the windows not being barred/locked is unlikely to be proved as grossly negligent, as there was a plan to remove stock in a couple of years, a need for passengers to open the windows and no other operators in a similar situation immediately removed their relevant stock from passenger service.