Rear-mounted ditch lights on 3R locomotives

In several photos and videos of NS engines, and during my railfan trips, I've noticed that many of NS' units, from SD70Ms to GPs, have ditch lights on the long hood side of the locomotive. I have a friend who has a MTH NS GP38 with both front and rear ditch lights. My question is, did these lights come standard from the factory? If so, does MTH put them on all NS locomotives? If not, where is a good source for these ditch lights, and how would I integrate them into the electrical system on a PS2 or PS3 engine?