The Auxiliary Power Contactor Box (APCB) is used to isolate the auxiliary power converter from the rail power while the substation is performing the power on test. It provides high impedance load to the input line to prevent substation from calling a faulty impedance on the line. When power is on and after a predefined delay, it turns on the internal contactor to give power to the auxiliary power converter. Also, the APCB has enough energy storage to power the internal contactor when rail gaps occur, avoiding undesirable cycling on the power contactor.