It sounds very interesting
For the story we can probably say that there is someone important in the train, like a president or high politician ?
It is also possible that at the end of the mission the bad guy is saying that the train was just for detracting from some other crime, like the invasion of the police headquarter or so, which is the second mission then.
May be the story should be more science fiction though, it involves hover bikes, but other than that there is no connection to the current Zone. The planet where it happens could be a holiday planet, or the player just wanted to join a meeting of the company he is working for (Sweep Fleet etc. are companies which hire soldiers for getting control of the planets).