Most motion sensors work by using passive infrared (PIR).
The PIR sensor works by sensing changes in the background IR (thermal) spectrum. So anything with enough heat to emit IR will be detected by the PIR sensor.
I don't know if our trains carry enough heat to be detected.
The only way to focus the IR detected area is through the white lens that comes with the motion sensor assembly. These lenses vary by application. Some are narrow, wide, high, low, etc.
Example: if you don't want your pet to set off the motion detector you choose a lens that blocks areas close to the ground.
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