Interesting that Top 40 was less mainstream than one might think in those late great heydays (or decades) of the format, in the 50s and 60s.
Most of the stations listed there were small, and some were even tiny. The early Top 40 "giants" were lesser signals on AM dials. The big signals were mostly devoted to "middle of the road" or "MOR" programming: chatty talk, pop standards, news and farm reports. ABC broke ranks when it went top 40 with its giants in New York and Chicago: WABC and WLS. WMCA in New York, WPGC and WEAM in DC, KQV in Pittsburgh, WIL in St. Louis, KHJ, KRLA and KFWB in Los Angeles... these were all secondary signals in those towns. In smaller cities like Roanoke, Syracuse, Santa Barbara and Winston-Salem, the top stations were as small as they get. WROV, WOLF, KIST and WAIR, respectively in those cities, were all on "graveyard" channels (1230, 1240, 1340, 1400 and 1450), all limited to a max of 1000 watts by day and 250 watts by night. And, since those channels were the most populated, "skywave" interference from the rest of the stations on those channels shrank coverage even more.
FM changed everything, of course. And now both satellite radio (SiriusXM) and Internet streaming are to music radio like vultures to carrion.