Train Spring
Some train stations have large horizontal springs at the ends of the tracks. These are safety devices to stop the train so that it will not go plowing through the station in the event an engineer is too busy texting. You would like to know how fast a train could be going and still be safely stopped by the spring. It looks like the spring is about three meters long. The maximum safe acceleration for passengers while stopping is about half of g. You know from years of reading books about trains that a typical train has a mass of 5E5 kg.