The Ngineering single led board works great but needs DC to power it; same for the 3-led version but putting together the leds for it in the way suggested in the instructions requires a very high level of soldering skill (which I have not got). Ngineering sell a very small DC power source that can handle up to about 14 VAC input. There are other DC power modules that can handle AC input; GRJ makes a number of these.
3rd Rail used to use a “blinker” led circuit in their diesels equipped with Mars lights but now have adapted the Ngineering single led module. I expect Atlas just used a blinking led. Not very convincing when you’ve seen the real thing or the more prototypical lighting effect boards available. Lionel and MTH have put some very convincing Mars lights in some but not all of their models.
There was a Forum thread a while back about exactly when a Mars light was used and how by various railroads. Practice was not uniform but for many streamliner diesels the Mars light was at the front above the headlight and intended to alert oncoming traffic or any that crossed the track. Most 3rail O gauge diesels simply have the Mars light activated when in forward motion.