If you aren't certain within a couple of centimeters of where you want a gap, cutting and filling it `after the fact' is the best way to go. By filling, I mean inserting something that will keep the rail separated forevermore.
Personally, I use insulated joiners, but that's driven by a couple of factors that probably don't apply to most modelers:
No matter how you open a gap, and how you keep it from closing, make certain that the ends of the rails are in perfect alignment and that you don't cause a kink to form.
Chuck (Modeling Central Japan in September, 1964 - with trackwork as bulletproof as I can make it)
I much prefer to gap. I don't like the looks of the plastic joiners and they take a lot of work to insert so that the rails ride on them without a bump. If you were to file the tips of a cut length so that you can leave an acceptable gap at each end, or use a jeweler's file, or a razor saw (they all work, as I have learned and use), it is quite reasonable to just leave a gap. I have never had a gap close, not on my Fast Tracks turnouts with jeweler's saw hair-thin gaps, and not on any I have done any other way. If you are concerned about unwanted closures or contact, just file the rail end on one side of the gap to increase the gap a smidgen, and then use something like CA or Gorilla Glue to fix the rail tips apart. I have never resorted to that, though, as I have said. My gaps just don't close up...not for any reason.
Crandell
Hello. I am new to n scale model railroading and have started out using insulated rail joiners to isolate a track section. What are the thoughts about using insulated rail joiners versus gapping the track? As my track layout gets more complex, it seems like the better approach would be to gap the track as I am a bit uncertain where exactly to isolate the rail when assembling and joining the track together. What are the pros and cons of cutting gaps in the track? Is it relatively easy? Thanks for any input you provide.