^^Here's the deal straight from Dollywood's website:
"1989
The $8 million Thunder Express roller coaster was added for the fourth season. (Thunder Express was retired in 1998, making room for the Tennessee Tornado which opened in April 1999.)"
^Thunder Express was a mine train coaster, but its top speed was 37 mph, much slower than TNT, and not nearly as tall on the hills. The names are coincidence only, I think, since Thunder Express was installed 4 years before TNT.