It stands for Texas State Network. I remember once when I visited them when they were in Ft. Worth at KFJZ. I was running at the time MRN, the Mississippi Radio Network. I was impressed. They actually had separate news writers. At MRN, the announcer did his own reporting, writing, etc., but still it was mostly rip-n-read.
TSN briefly had a competitor, IBC (Interstate Broadcasting Company) which also had Louisiana and Mississippi state networks. IBC lasted less than a year. The investor pulled the plug.
There's a little bit of TSN's hsitory on its website.