Whenever you 'work' treated timber you're effectively breaking the seal, as the chemicals only sink about 6mm deep all round and any cut or planed areas will need re-treating. As Brian and Simon have said, it's best to do all the machining first on un-treated softwood and then send it off to the vacuum tank. :wink:
Mike, you're quite right there; creosote was banned in May 2003, if I remember correctly. Most places aren't even allowed to sell the old sleepers, although I believe you can still pick them up from reclamation yards. Even with the "new" sleepers (beech is the main one, although oak and azobe are also used) you've got to take some care as they'll sometimes bang great lumps of metal in to the end-grain to stop the green wood from drying out too fast.