Yes, I'd seen them and others like them. That's why I thought I might try it.
So it seems that you can't crouch to get under the bridge and must despatch them before you get there. As the sniper rifle won't do it fast enough, needing 3 or 4 head shots to kill each of them, it must be the AK 47 or the M4 (or the minigun)
Some of the methods of getting onto the train after the first bridge look interesting, especially the one where you get onto it as it goes into the first tunnel
Edit: Ah well, I've given up
It's interesting to try different techniques but I really can't see any advantage to doing this mission this way. If I take Smoke with me and ride alongside the train we can kill all of them (very nearly) every time.
With the jumping on the train I've tried it 20 or 30 times and haven't managed to kill them all on a single one of those attempts. I can get onto the train easy enough but even if I walk up close to them they are just too well protected and I can't kill them before I get swept off by the bridge.
Riding ahead of the train and going up above the first tunnel might be worth a try. But if you go too far ahead of the train then you lose the mission.
I notice that if you ride ahead of the train the the car that's parked across the tracks doesn't appear there until the train is quite close to it.
I also noticed that there are 4 Vagos on the train when it leaves the station but by the time it gets to the level crossing there are only 3 of them, even if you haven't been shooting at them