NAT Track width
BOAC,
I'll try and answer your question ... though this isn't gospel, but is formed from my previous experience as an ATCO.
The width of a NAT track is 0.01 nm. Its a conceptual width, because its not the same as an airway in the UK, with a width for example of 5nm, or 10nm.
A NAT Track is not an airway - its a line.
The intention, and hope, is that you fly that line as close to being on it as possible.
Because of weather, SLOP, In trail turbulence, Nav errors, and more, an aircraft will at times not fly on that line - and there are tolerances associated with the above (10nm, 2nm, 2nm, 25nm, etc.).
Hope that helps ...