The GE/GN8532 regulations state that the RFS should be kept in their original, marked locked boxes and stored in a suitable locked cupboard, store or container that is only used for the purpose of storing explosives and their connected implements. The safe storage of such is required to followed to ensure they comply with the MSER and ensure accurate control and record keeping arrangements.
This doesn't mean putting them in a box with as small a padlock as possible and shoving them on some random shelf in a building accessible to any number of Network Rail personnel on a daily basis.
The same with vehicles, The Carriage of Dangerous Goods and Use of Transportable Pressure Equipment Regulations 2004 requires that they are carried and stored in a cargo container or locker as part of a separate load compartment and while you can get away with putting the original sealed package in the boot of a car (or back of a van) inside of a padlocked steel box, you cannot store it there if you are not in attendance unless the vehicle is secure and the box is mounted securely onto the cargo compartment or locker. The later being synonymous with security cage.
If what you say is actually happening then Network Rail are responsible for not securing dangerous materials securely and safely.