Trains do not simply slam into the back of other trains. The recent incident in Spain occurred because the system that would have prevented that was broken, so they turned it off (as it would not have allowed the train to go while it was broken). This reminded the whole world of train operators why they Do Not Do That Ever Ever Ever.

Have you been on a train station and heard an announcement that a train is delayed due to a signaling fault? That is because they are not allowed to ignore a signal even if they know they are broken. The signal must be repaired, no exceptions. When you create an exception you get a train crash, like in Spain, so the rule is No Exceptions Ever.