When two vehicles or convoys collide (except the train in a road/train collision), the vehicles will explode into a fireball. This occurs even if the vehicles aren't carrying flammables of any type, such as an electric passenger train.
The same happens if a truck, bus or anything else goes off road or if you sell the track or road under a train or road vehicle, unless its broken down.
Wide Open Sandbox: Once you beat any scenario, the level opens up for you to do whatever you want. Also with the included Level Editor, you can make your own.