If the box is primarily white with yellow & aqua trim and red lettering, it's from the 90s, or maybe earlier. The newer ones are primarily silver with black & red trim and red lettering.
Here's the Atlas webpage on the newer versions from 2003 on. I'm assuming HO since you didn't specify scale.
Tom
https://tstage9.wixsite.com/nyc-modeling
Time...It marches on...without ever turning around to see if anyone is even keeping in step.
I purchased an Alco RS1 (Southern) from ebay. It is an Atlas Locomotive but I was wondering how to tell an estimate of how old it is. It's in the box and hasn't been used but the Atlas box looks kinda retro and the locomotive has hook horn couplers. The paint looks very good but looks still, it has an aged look.