First mentioned on here at http://www.railforums.co.uk/showthread.php?t=34017 they have now been published in full as both a paperback and e-book:
http://www.amazon.co.uk/Railtrack-and-Other-Letters-ebook/dp/B00CMCCHOG
http://www.amazon.co.uk/Railtrack-and-Other-Letters-ebook/dp/B00CMCCHOG