Grind along the rails, as was said. When you reach a part where there is a pylon on the left and right, but not in the middle, IMMEDIENTLY before the first construction site bit. Go on the left track, and strafe jump to the left right before the pylon. The electricity shooting off the sides will hit you, (as long as you haven't put a shield on, which hopefully you haven't. when it hits you, glide to the left of the tracks and onto the grass. Then keep walking along the road, and when you come to a split, turn right. continuing along the road, you should eventually see a building in front of you. once you come up to it, on the left hand side of it is a strip of land going along the edge of it. jump onto this, and keep walking. eventually you will fall through the floor. turn your right and walk a bit. you should be being shot at. jump, and you will go through the narrow ceiling. you won't be able to see. enter look mode (L1) and you will see ahead of you three thugs trying to kill you. Kill them, and the ones that spawn after them. Then, behind them is a portal. Step onto it, make sure it says "Go to Insomniac Museum", as there is another portal around here which returns you to your ship. If it is the right portal, press triangle. You will blast off to Planet Dantonia, which is actually Earth, if you bother to look!
Once here, you can see stuff that was cut out of the game, edit the explosions and laser blasts in the game, see the testing the room where they checked ratchets jump and walk distances/levels (if you can solve the electrolyzer puzzle) and lots more. I haven't found the "make your own infiltrator puzzle" thing the other guy said about... but when i can be bothered ill look for it!