One way you could solve this problem would be to install light sensors under the approach tracks at each side of the crossing. When the train passes over the sensor (thus blocking the light), an electronic switch is engaged that sends power to the crossing gate motor(s) making them lower. Once the train passes by, light reactivates the sensor and allows the gates to raise.

To explain, the sensor essentially acts as an on/off switch which you would install between the power supply and crossing gate motor. When a light source is present, the sensor would be set up to be ‘off’ i.e. no current to the gate motor and when the light source is absent, the sensor would be ‘on’.

Hope this helps,

Geof from the US