Ansaldo STS / Union Switch & Signal Inc. PD-1 DC Biased  Vital Relay.

It was pulled from working condition. It shows signs of use.

We have a large number of these in stock, please contact us if you are buying in bulk. 

Original Manufacturer Description:

A biased relay is equipped with a permanent magnet and arranged so that the  relay armature picks up only when de current is flowing in the coil in a  prescribed direction.

The AAR defines a biased relay as: "A relay that will operate to its  energized position by current of one polarity only, and will return to its  deenergized position when current is removed".