Train accidents are rare. The rate of train accidents that resulted in passenger or workforce fatalities has been less than one per year over the last twenty years. Because they are so rare, we can only understand the risk properly by looking at the underlying things that might cause them. These precursors can indicate the risk of accidents happening, even though they do not often result in an actual accident. RSSB has developed the Precursor Indicator Model (PIM) to quantify changes in that underlying risk over recent years.