The K-Line Porter, is it a diesel or steam locomotive?
On some diesels you need to look at the ends above the knuckle couplers for a hidden screw, on steam locomotives there may be a screw in the smoke stack that is holding it in or in the cab area or the frame may need to slide forward like an American Flyer loco, Lionel put a shaft through the loco on some of their steam locos. If a steam loco the side rods may need to be removed by taking out some screws.
Did your Porter by chance have the front screw under the boiler come loose and then you tightened it? If so, I suspect it is out of time…the side rods are not correctly aligned. The bottom side of the front boiler has a single screw that holds the whole front of the lower assembly together. If you loosen it, you would see that the steam chest and all come down. You have to play with it a little to get it in correctly. Afterward, put either Blue loctight on the threads of the screws, or some clear nail polish on the screw head to frame to secure it in place.