1. An on-board device that is equipped in a train having
a battery capable of supplying electric power when a
vehicle power supply is off, a parking brake to fix a wheel
when the train is stopped, and a parking-brake controlling
electromagnetic valve to be energized by power supplied
from the battery when the parking brake is released, and
that retains position information of the train stored when
the vehicle power supply is on, even when the vehicle power
supply is off, the on-board device comprising a parking-
brake release detecting circuit that includes:
a parking-brake-controlling-electromagnetic-valve energization detection unit to detect energization to the parking-brake controlling electromagnetic valve when the vehicle power supply is off; and
a parking-brake-controlling-electromagnetic-valve energization detection storage unit to store therein a result of detection until stored contents are reset.
2. The on-board device according to claim 1, wherein
one end of the parking-brake controlling
electromagnetic valve is connected to a wiring line that serves as a path of power from the battery when the parking brake is released while the vehicle power supply is off,
the parking-brake release detecting circuit includes first to third relays and first to fourth contacts,
one end of each of the first relay, the first contact, and the second contact are connected to the wiring line,
the first contact and the second relay are connected to each other in series,
the second contact, the third contact, and the third relay are connected in series in this order,

the first contact is turned on when the first relay is energized, and is turned off when the first relay is not energized,
the second contact is turned on when stored contents of the parking-brake-controlling-electromagnetic-valve energization detection storage unit are reset,
the third contact is turned off when the first relay is energized, and is turned on when the first relay is not energized, and
the fourth contact is turned on when the second relay is energized, and is turned off when the third relay is energized.
3. A display device equipped in an on-board control device included in the on-board device according to claim 2, on which an indication that the position information is invalid is displayed when the vehicle power supply is turned on while the fourth contact is in an on-state.
4. A method for detecting parking brake release using the parking-brake release detecting circuit included in the on-board device according to claim 2, the method comprising:
a step of supplying power to a wiring line that serves as a path of power from the battery and energizing the first relay when the parking brake is released while the vehicle power supply is off;
a step of turning on the first contact and turning off the third contact by energization of the first relay;
a step of turning on the fourth contact by energization of the second relay, and maintaining an on-state of the fourth contact until the third relay is energized; and
a step of turning off the fourth contact by

energization of the third relay caused by turning on of the second contact.