A ballast tamper is a machine used to make railway tracks level. It is like the railway version of a road roller apart from it lifts the track out of dips instead of squashing the parts where it is too high.
Before tamping machines were made, tamping was done by lots of people with shovels. They make tamping quicker and the track is better than if tamping is done by hand. Also, some new track is too heavy to be lifted by hand, so a tamper has to be used.
New tampers can also move the track from side to side to make it smoother.