Commonly human error, either made by the driver or by the signal operators. Also accidents caused by mechanical failure, track failure, signal failure, anything unwanted falling on the line...
The cause of an accident is when a situation or conflict arises purely due to randomness and when conditions are more likely to support the accident. If there is harm, damage, or injury sustained, it...
the shock wave from the hit or even a very strong whip lash go through the skull and reach the brain. The nerves and cell are very tender so even the lightest damage can cause disorders.