They weren't mounted, they were kept in a nearby bunker. The charges were standardized round flat 50kg lumps, called cheese charges because they looked like a wheel of cheese. The roads and bridges had prepared openings looking like a normal gutter (round in germany) into which the cheese charges fit perfectly. The top plate could often be recognized because there was a single number stamped in, telling you how many cheese charges to load. What was always present however was the initiator charge and the wiring to the nearby bunker.

There were even more interesting things like the new Elbtunnel in Hamburg having massive blocks of concrete above all entrances. The girders holding those blocks were intended to be cut by explosive charges drop the blocks, blocking the tunnel.