I recogonized the similarity of S X to D T imeadiatly when I first found Symphony X and I was linked them on Amazon where it said people that have purchased Dream Theater have also purchased Symphony X, I thought wow thats cool and upon listening to samples I began my collection. There are differences but very compatable. I actually assumed they were from Europe due to the neo classical sound but was even happier when I found they were based in Jersey.
The Transiberian stuff stemed from Savatages Dead Winter Dead LP where they used pieces from both Mozart and Beethoven
So theres two I guess three that I knew of
Savatage - Prelude to Madness - Hall of the Mountain King
Savatage - Mozart and Madness & Dead Winter Dead from - D W D
Queen - Bohemian Rhapsody (the choral part) I knew it came from somewhere but just yesterday I heard a Mozart tribute put together by one of the later composers that contained a few of the melodies
Crack the Sky - Did a cool live jam that included some of the Nut Cracker Suite in the middle of Surf City (I think). Disclaimer - I recommend this band cautiously to the younger or metal crowd, is has to be select songs. They were kind of an avant guard, alternative, tongue in cheek, progressively confused, hardrock band in the mid 70's. Their first self titled work (1975) was voted best new band or something along that lines in Rolling Stone and is all great to my old ears. Their 3rd studio album Safety in Numbers '78 has a song on it called Nuclear Apathy that is a must hear in my opinion. The second Animals Notes was actually a concept album but a bit bazaar for most with nothing heavy in it, kinda had a Broadway feel. They did a few during the eighties too but its all pick and choose, lots of off the wall stuff with a few great songs. Anyhow
They did a few classical approaches as well as the Nutcracker Suite jam