Re: Four Lane with "Ghost Racer" sections:
Darn.
Well then, in that case CB, I would have to figure out how to cut a bump sized divot out of a straight track. It will not be easy, and it very well may take ruining several straights. But with some combination of a straight edge, razor knife, and a small, possibly razor or coping saw, fine toothed metal file, sandpaper, and maybe a dremel, I believe I could do it. The trick, of course, is to have your modified straight snuggle right up to your "Ghost Racer" piece.
Then I would fill the hole in the track.
1) Get a cookie sheet or flat piece of metal and tape down some waxed paper on it.
2) Put clear packing tape on the topside of the track. Trim the cut part as necessary. Make sure the packing tape also covers the uncut edges of the track.
3) Mix up some bondo and spread it under the track. Up to the cut edge, of course, and probably an inch or so inward toward the center of the track.
4) Push the piece down on the flat sheet. Put more waxed paper on the bondo and mush it into approximate shape on the edge.
5) Bondo hits pretty quick, Carefully lift the piece off the sheet with the waxed paper attached, and then remove the waxed paper from the track.
6) Probably the bottom of the track is close to being close enough, so now you are only dealing with the edge of the bondo in the modified part of the section. Sand the high spots off. Fill the low spots using the same technique if necessary. Repeat. Remove the clear tape from the topside when it is getting close. Try to keep the racing surface bondo and scratch free if you can.
7) Paint the track with brush or spray as is appropriate. I like Krylon Fusion flat black. Hopefully it will just be the edge of the track, and even that will be covered by the adjacent "GR" piece.
Now, the guy on the modified track has to run over two diagonals and a chuck of the GR track. Clearly that cannot be a lump, it has to be smooth. I would consider tying them both down to something--the plywood below if it is permanent, or a very thin piece of something if it is a temporary setup. Shim and/or sand with very fine sandpaper (if you have to) to get the racing surface as smooth as you can. It should be like any other joint that the tires pass over when you are done.
There may well be a more elegant solution to "filling the hole" that other members suggest--and if so I would take that route.
But that would be how I would attack it.