I asked that of one of the staff on my last journey. There were more high speed chargers being put into the shops car park area. Clearly the higher power chargers stir the molecules more aggressively and heat them up so being more likely to trigger an event if there is a weakness?
His answer was - smoke detector plus in car cameras - put your foot down so it does not stop mid tunnel unless extreme.
It appears that as they take a bit of time to turn from smouldering to intense almost unstoppable conflagration that it allows them to get out the other end and into a siding. Putting them on the lorry carts would be bad as it would not contain the heat and could damage the tunnel more.
Not certain if it is true but that was what he told me. Makes sense it is not that long a run through and the alternative of a chemical fire in the tunnel sounds a lot more difficult to deal with given the fumes and access.
