No contract, receipt, rule, or regulation shall exempt any transportation company from the liability of a common carrier which would exist had no contract been made or entered into and no such contract, receipt, rule, or regulation for exemption from liability for injury or loss occasioned by the neglect or misconduct of such transportation company as a common carrier shall be valid. The liability referred to in this section shall mean the liability imposed by law upon a common carrier for any loss, damage, or injury to freight or passengers in its custody and care as a common carrier.