The DXer's dubbed them "Graveyard channels." That was because each of those frequencies was home to 300 stations in the lower 48. This makes picking out any single station almost impossible. If this sort of challenging DX’ing appeals to you be sure to visit the Graveyard DX blog here. It's inactive, but very telling. In the US, Canada and Mexico stations on the AM dial are spaced at 10 kHz intervals.
In 1960 the FCC finally admitted that some of these small communities had grown larger than the coverage areas of their local stations. So, the suits allowed all stations on the six aforementioned frequencies to increase their daytime power to 1000 watts. This was modified in 1982 to permit the higher power 24/7. the assumption was that they will only interfere with each other. Which they do. Canadian and Mexican followed suit.The result? Congestion is very common, and DXers may hear dozens of stations on a single frequency. As Fybush once wrote :
"Interference... in AM broadcasting is cumulative; Just listen to any of the "graveyard" channels... to hear what it sounds like when 300 stations are each throwing a kilowatt into the ether. "
Now, most stations on the six "Grave yard" frequencies operate at the permitted max of 1000 watts, even at night. A small number of them have had to use directional patterns to suppress interference to each other. You still hear cross talk everywhere in American essentially until you're in their parking lot.