For high-production installations significant advantages can be realised by installing a four-rail system like that shown. The rails are installed at the same centres, and trolleys can run on the centre pair or on the outer pairs as required.
This system allows work waiting to be blasted to be loaded onto trolleys on one pair of rails, while the work being blasted is carried by trolleys on the other rails. When blasting has finished the blasted work can be wheeled out and the unblasted work wheeled straight in. This allows blasting to proceed almost continuously, ensuring maximum output from the blast chamber.