We put a RR spike on A line tracks between Broad Channel and Howard Beach one day thinking it would flatten out huge. We were just kids and didn't know words like "malleability" or "ductility".
Didn't derail the train, but the head car jumped up about a foot off the rail and came down with a bang that you could have heard in the BX.
Anyway...
Massive derailments happen when the train is moving pretty fast, one car comes off the rail and there's a chain reaction through the train.
Slow speed it's usually pretty minor.