from my friend -

Round timber poles were called rails as in horizontal fence rails. The song may also refer to the several rails that would be use instead of a gate. They would be used removing (slipping) them in and out of position when access or closing access was required to a paddock. At present there is a Facebook group entitled Tales From the Top Rail.

& adding tho Gerry's post - copies of 2nd edition of Chris's book are still available - friends on the committee have several boxes in their custody!