I don't think there's any rhyme or reason for how far apart they are, just that each manufacturer designs it slightly differently.
For example, on the MBTA commuter rail in/around Boston, the bi-level Hyundai Rotem cab cars have the headlights a bit separated, but the older Kawasaki cab cars are closer together. The single-level Bombardier cab cars have separated headlights but the slightly older MBB ones are closer together.
Keystone ex-Metroliner cab cars have the headlights spread apart. SEPTA Silverliner EMU cars made by Rotem have lights somewhat close together but the GE ones are further apart.
I don't think there's a pattern.