That machine is a tamper.
It rides on the track, which has perviously had ballast dumped upon it. The tamper lifts the track and ties, and uses vibrating forks to evenly tamp clean ballast under, and between, the ties. The ballast and the track sit on the sub grade. Automated tampers like this can add superelevation on curves. The little cart it is towing is used as reference for lasers that balance the machine to keep everything level and straight (or elevated and curved). Those measurements are called cross level.
The process in its entirety is called surfacing. It smooths out the ride quality of the track, and -- very importantly -- provides drainage through the clean ballast.
(Underlining provided for railway engineering terminology)
