Vehicles can have accidents, except for ships, and here is how this may happen for each vehicle type.
Road vehicles can’t get into accidents with each other, but they can be destroyed in collisions with trains, although the train will continue on after such a collision as though nothing had happened. Now, despite the fact that the lights on a railway crossing flash and stop any approaching road vehicles when a train is coming, if a vehicle is already on the crossing, either because it is slow or broken down, and the train is moving quickly, the road vehicle might not be able to leave the crossing in time and will be… obliterated. However, path signals can be used to close the crossing earlier and thereby reduce the chance of collision. Also, if a double track railway is in use, make sure there is enough room for the road vehicles to wait between the tracks. Alternatively, a bridge or tunnel can be used to cross roads and railway tracks instead, making road vehicle collisions with trains a non-issue.
Aircraft will sometimes crash when they land at an airport which will block the runway for a while and lower the station rating at the airport considerably. The size of the plane is important because planes, by default, have a 0.07% chance to crash* each time that they land at an airport. This chance is increased to 5% if the plane is large and trying to land in a small or commuter airport. However, the amount of random plane crashes can be changed via the advanced settings menu like shown below. The options for this are Normal, Reduced, and None.
It is possible for trains to crash and this will destroy both trains, render the track impassable for a while, and significantly reduce the station rating at all nearby stations. The solution to this is simple – make use of signals.