Why Every Locomotive Has Three Headlights

If you have ever stood at a railroad crossing at night, you know the sight. Three white lights in a triangle, one bright beam up high, two smaller ones down low, all of them blazing through the darkness toward you. It looks like it has always been that way. But for over a hundred and sixty years, every locomotive carried just one headlight. So why did it take until nineteen ninety six for someone to add the other two?