Ride the grindrail is a mission in Ratchet & Clank, taking place at the Gadgetron Headquarters on planet Kalebo III. After entering the lobby, Ratchet and Clank found the grindrail leading to a room where they obtained the Map-o-Matic.
When you are in the main lobby, take the path to your left to find an elevator that will take you up to where the grind rail is. Defeat the two blarg bombthrowers that appear behind you, then jump here to begin a rail section, where as with previous grind segments, 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 you will need to hit with the wrench. When you see the first, jump to the rail on the right and strike it, then quickly jump back. When a rail appears to the right straight after, jump to it to follow 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. After you have hit the 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.
Upon jumping off the rail, you will reach the Map-o-Matic room at the end here. Head into the room and break the glass to obtain the gadget. Before you leave with the taxi, head to the right to discover a teleporter that will take you to the Gadgetron Break Room. This is a room that is full of sheep; obtain the holocard set in here and teleport back out, then take the taxi back to the Gadgetron Lobby in the starting area. If you have not done so already, complete the main mission "Find the Focus Lab". 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 gathering RYNO holocards (of which all but one are available before the Deplanetizer) and all telepathopus brains before the final mission, "Locate the Gravity Stabilizer".