I think the key is the second of the recommendations - "The
RAIB▸ has made two recommendations to Network Rail. The first addresses the risk of a runaway being initiated by debris on the track causing multiple disruptions to the braking system on this type of train. The second recommendation addresses the possibility that similar risks might be present for other short formation trains that operate on its infrastructure".
The braking system on these trains will be made up of standard freight waggon braking components, I doubt if the distributors and reservoir's are located anywhere unusual.
.............. (Its been best part of 40 years since I did a train braking course
)Train air brakes rely on a res on the individual waggon / coach to apply the brake via the brake cylinder (unlike lorry brakes that have springs and air is used to hold them off. The distributor valve is a proportional valve that converts the drop in train pipe pressure to an applied pressure to the cylinders.
