I've recently been getting into some reading about PAS 55 and was wondering if anyone knew where i could obtain a listing of the 21 key clauses involved with PAS 55. I've been reading that PAS 55 incorporates these 21 clauses but have not been able to find what these 21 key clauses are. Any guidence would be greatly appreciated.

Check this:
www.rail-reg.gov.uk/upload/pdf/lloyds-rail_signalling-sica_assess.PDF

http://www.irse.org/support%20files/YM-HalfDay/Gab.ppt