Well Dark Reign was 1997, and its sequel came out in 2000, so it was getting to be old already in 2004 - even if I had never head of it before this thread.
It played during the install and then again during the start of the game but you could disable it as I did. It gave you something to watch during the very long load time of TRS2004.
I think that was done in the trainzoptions.txt file by adding the line: = Introdisable or something like that. I never did because my cat used to watch the intro.