Couple of excellent articles on the GWR website.
http://www.gwsr.com/news/features/steam ... -0sts.aspx
http://www.gwsr.com/news/2009/may/earl- ... avid’.aspx
(NOTE: the second link needs copying/pasting into your browser - the 'earl-david' part of the link isn't recognised as an url by the forum software due to the commas!)