Ride the rail is a mission in Ratchet & Clank (2016 game) taking place in Fort Krontos on planet Batalia. Clank picked up a message for them, found in an infobot reached by riding a rail with the Grindboots.
The rail is located at the end of an upwards leading slope to the right of the main path in the starting village. The first rail is a short loop that takes you back to the start; rather than riding it, hop to the rail on the left and ride this one. On this next rail, dodge obstacles across this rail by jumping over them, or hitting the mines off, while jumping over gaps. When another rail appears, jump across to it.
Eventually, the rail path should lead you down through a mountain and towards a platform on which an infobot is located. Make sure to jump when heading off the rail to land on the platform and use the infobot. This infobot will present a transmission from Grimroth Razz about his brother on Pokitaru, giving you the mission "Meet up with Felton Razz". This should be the next planet you go to.