I have found that, when using a Dremel to cut the track, it's best to have the tool over the track that you want to use, rather than over the bit that you're cutting off. The cutting disc is usually smaller than the diameter of the tool body, resulting in a slight, off-vertical, angle to the cut. If the joining piece also has this slope, the other way, then there is an inevitable gap at the top of the rail, even though the bottom edges of the rails are butted up hard.