It's right in the name. Any time a train car or locomotive has its wheel leave the track, a derailment, it is counted as such. Long ago I worked in a facility that had rail for shipping and receiving materials, which oddly enough included vinyl chloride, and we were all given training for the safety aspects of having rail run through the facility even if our jobs did not involve rail at all. In the years I was there, we had quite a few derailments, but none were serious because the majority of them were simply a tank car having its wheel trucks jump the rail. Those incidents had to be reported as derailments because they were derailments. So yes there thousands of derailment incidents annually. You don't hear about them because the vast majority of the derailments are extremely minor and do not result in spills, injuries or deaths. They are the fender-benders of the train world. Just like with cars, we only really hear about the twenty-car pile-ups and not the minor accident that resulted in a small dent. Thousands of minor vehicle accidents every year but how many can you recall making the national news? This make more sense now?