Here is an excellent video on having expansion gaps between two sections of flex track. Note that the rail joiners are not soldered which allows the rail to expand and contract without causing kinks or pulling the rail from the spikes. He uses a .019" feeler gauge but my calculations indicate that a .015" gap would be sufficient on a 30 inch length of flex. That is .015" on both ends of the flex track. Here is the video: Perfect rail gaps with N scale flex track on your model railroad layout for better operation - YouTube Notice he gaps only one end of the track. There should be a corresponding gap between all track sections.