Some services, like Amtrak, require that a ticket be purchased in advance, specifying a specific origin and destination. Since the train's crew know what tickets were sold, they also know where people are coming from and going to, and they simply stop only at those stations required by the tickets. Services that don't have advanced ticketing, or which sell tickets for a range of destinations or travel times, require ways of knowing whether or not someone is waiting at a station or platform. These may range from a passenger speaking to a dispatcher on a phone located at a station to simply pressing a button to activate a signal such as a flashing light somewhere before the station that the driver can see in time to slow down safely.