That's a tough one. I think you should ionize all the cars on the train except the ones that don't have guns, this will slow it down enough for you to go on a killing spree for ties, at-sts, barracks, missile turrets, and turbolasers. It's difficult cause they don't give you alot of time, i think only 4 minutes? btw, you CAN destroy the train station that the train goes to when you fail the mission, but it's not really necessary.
"Without strife, the victory has no meaning. Without strife, one does not advance. Without strife, there is only stagnation."