I have an Atlas Southern SD35 from the same time frame (before they added TAS cruise) and you are right about the horn. To my taste, it is the most realistic horn that Lionel has ever offered.
I run mine with a Legacy SD40 set at medium momentum and use the Atlas a the lead engine. The legacy is forgiving enough that they actually work very well together and that way I get the sound I like and speed control. Currently, this is the only non cruise engine I still have on my roster.
About the horn: I was in Charlotte getting gasoline in an area close to an older residential area. As I was pumping gss, I heard a train horn behind the station. Evidently, there were tracks running through the neighborhood and the engine sounded crossing signal at each block. I drove into the neighborhood to see the engine and it appeared to be a GP7 or 9 and I thought the horn sounded just like my Atlas engine at home. When I got home, I ran the engine and was amazed that the horn sounded so correct.
Happy railroading,
Don
