A crude measurement:
Zooming in to some 1:25k mapping, I measure the 40mm grid as 140mm, and the width of a BOAT track as about 1mm
That gives the width of the track as 40 * 1/140 = 0.29mm
The edges of the track are a bit blurry at that zoom level...
I'd guess at 0.25mm, but the width does seem to vary, at least at the bit of map I'm looking at.