Ride the grindrail is a mission in Ratchet & Clank (2016 game) taking place in the Gadgetron Headquarters, Kalebo III. After entering the lobby, Ratchet and Clank found the grindrail leading to a room where they obtained the Map-o-Matic.
Head up the elevator on the path from the left in the main lobby to find a grind rail. Defeat the two blarg bombthrowers that appear behind you, then jump here to begin a rail section, where as with previous sections, you have to jump across rails to avoid obstacles and hit mines off your path with the OmniWrench. Several grav-trains appear, which are most dangerous and should be dodged immediately.
Across the path, you will encounter several buttons hit with the wrench. When you see the first, jump to the rail on the right and hit it, then jump immediately back. When a rail appears to the right straight after, jump to it to a new rail path, hit two of the buttons here, then jump back left to the main rail path. Make sure you jump across the gap, and keep following this rail past the trains, and when another rail path appears on the right leading to a gate blocked by an orange forcefield, jump to it. If you hit three previous buttons, hit the final button here and the forcefield will deactivate, leading to a rail path with a gold bolt. Collect it, and then jump back to the main rail path. Jump across the gap to the final stretch.
You will reach the Map-o-Matic at the end here. Break to glass to obtain the Map-o-Matic inside. Before you leave with the taxi, head to the right to take a teleporter to the Gadgetron Break Room. This leads to a room full of sheep; obtain the holocard set in here and teleport back out, then take the taxi that will return to the Gadgetron Lobby in the starting area. If you haven't already, complete the main mission "Find the Focus Lab" if you haven't already. With the Map-o-Matic, you can easily return to other planets and obtain collectibles, such as RYNO holocards, gold bolts, or the telepathopus brains on Gaspar. Consider getting RYNO holocards (of which all bar one are available before the Deplanetizer) and all telepathopus brains before the final mission, "Locate the Gravity Stabilizer".