Not entirely sure about this, but I am led to believe that the sack on the hose cart contains the standpipe and tool for hydrant access. Typically, German fire services use a double outlet hydrant with individual valves so that two hose lines can be connected, including on occasion to feed separate pumpers. This also allows a supply to be started with a single line until the second can be run out, if it is anticipated that the entire hydrant capacity will be needed.
Added by Rob Johnson on 28 June 2023.