We show that the conductivity maximum, well known for salt-in-polymer electrolytes is not the maximum conductivity in the system when the salts used are chosen for their resistance to crystallization at high concentration. The polymer+salt system, with increasing salt content, goes through a transition zone characterized by a maximum in glass transition temperature. Beyond this point additional salt serves to plasticize the salt-crosslinked polymer system, establishing thereby a region of increasing conductivity in which the conductivity can reach very large values. We show that `polymer in-salt' solutions constitute a new type of rubbery solid electrolyte in which the conductivity is dominated by cation motion. Such systems are true hybrids of the previously distinct `superionic glass' and `polymer electrolyte' types of amorphous solid conductors.