This is pretty tough & best done in a cut scene. Though I have had success with doing this in a skirmish/multiplayer map.
What you see in the cut scene in the 6th mission of the China campaign for Generals, is really just a totally different object. What it is, is train bridge that has a train a little ways from the bridge. Both the train bridge & the train are part of the object. The object is called, SpecialAffectsTrainCrashObject located in the System folder for the Civilian faction. If you place this object on a map, you'll see a train just sitting aways from it. When the train bridge has lost all it's hit points, then the train starts moving, the bridge brakes down, & the train falls.
There are other objects like this bridge. The bridge that blows up with all the china tank on it is basically the same. When all it's hit points are gone, chinese tanks spawn on it & drive along it for awhile. Then suddenly, the bridge starts blowing with all but a few units making it across. Then the remaining units just disappear. Other examples are like the satellite in the 5th USA mission in Zero Hour, or the air show raptors in 2nd USA mission or 3rd GLA mission. These objects, however, have to be activated through a command button ability, not getting destroyed. To learn how to use these objects, exirement with them in new map. You can also check out how EA Games did it in their missions.
To use the train bridge right, place this bridge over some water because when the train falls splashes come from the impact. This wouldn't look to good over dry ground. You'll have to use trial & error to get the bridge at the right height so that the train stops falling when it hits the floor. (If you put the bridge to low, the train will fall right through the ground. If it's to high, the train will stop falling before hitting the water.) But don't have the bridge to high because for this to look good, the train tracks need to come out of a tunnel. Have this tunnel close enough so that the motionless train is covered up in a mountain. Either that or you could put the bridge close to the border of the map to cover it up.
Now you'll need to do some scripting because the original train will get in the way. You can just create a script to delete the train the moment the bridge is destroyed, but that wouldn't look to good if a player was looking at the train right when it happened. Though if you don't mind that, you need not read this paragragh further. You'll need to place some trigger areas so that after the bridge is destroyed, the train will get deleted off the border, or in the tunnel before the bridge.
Looking at the map I did this on, it looks like you have to make the bridge selectable through a script. I kind-of remember not being able to target the bridge with the default settings. Not even by checking, Selectable, in the Object Properties window.
To sum this all up, I've upload the map I made a long time ago that has this done, but then decided not publish it because map way to big! Check out the scripts I made for it. The map has a few other features too that you might be interested in.
Download Attachment: 
The Map.zip
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This would all be easier in a cut scene because then you can put the normal bridge in any place on the map. You wouldn't need it next to the border or a tunnel. Then at the start of the scene, the normal train would be deleted & the SpecialAffectsTrainCrashObject could just be spawned & then killed.