Nochange / Replacemodel / Remove / Texturechange.

Nochange: Nothing happens to the original model. You can still have explosions, shrapnel materials or awards, for some reason. Nochange is used for switches, because nothing happens to switch, but it can still cause a damaging force to some part of the track. (To destroy barriers in Nuke-mission, for example)
Replacemodel: The original model will be replaced with some other model, like in the city track the petrol pump will be replaced. Write these between "Number of run time variables" and "Reserved 1":
PetPump_broke		// New model
100			// Chance of fire/smoke (percentages)
1			// Number of fires/smoke columns
0,0			// Min,Max smokiness (0=fire, 1=black smoke, 2=grey smoke, 3=white smoke)

Remove: Just removes the original model. As always, you can have explosions, shrapnel materials, awards, etc.
Texturechange: See Econobrick's "Smashable glass"-chapter