That one follow the train mission is one of the easiest missions and I don't understand why it's widely known as a really hard mission. The very next mission after that one is tough sometimes. (that one where Smoke drives the bike and you shoot the guys tailing you throughout the drainage ditches or whatever those are.
I found the train mission really easy as well and the next mission on the bike a bit more difficult (although I wouldn't call it one of the tougher missions).
Flight school however was pretty tough for a storyline mission and I'd probably call it the toughest one (storyline-mission-wise). Rockstar shouldn't have forced the later missions of the Flight school to be mandatory because some of them are ****ing annoying. And I know quite a number of people who have given up on SA after failing and failing at the FS.
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Rock n' Roll and Glam Metal... It never died