Sensor track location relative to grade crossing

I suppose that's going to depend on how fast you run, and I suspect not all the locomotives will have the same tempo of the crossing signal, never really gave it much thought.

Keep in mind that it also depends what kind of locomotive you are running.  The sensors on the diesels are usually in the center of the engine.  The sensors on the steam engines are in the tender.  That means anywhere from 6 to 24 inches of train have crossed the sensor before it is even activated.  Add the additional amount of track you want open when the crossing gate or signal starts to activate and you may want at least 5-6 feet or more before the crossing.  Best to just try it at different distances and see what looks right.