It will depend to a certain extent on the size of rail. It is the base dimension that will control, since obviously they cannot overlap, and then you also have to allow more room for the fastenings, such as spikes or Pandrol clips. Possibly about 8" difference might be a bare minimum with spikes, and greater for clips.
Thanks, cx500.
Russian soldiers patrol by the Druzhby Bridge linking Russia and North Korea in January 2003. From Business Insider April 24, 2019
Sometimes you don’t wan’t trains stopping if at all possible (to open and close the switches)
and sometimes the brakie and the conductor don’t wanna get their widdle boots dirty or get out of the cab…[:-^]