Speaking from experience the clearence of trees from proximity to powerlines should where ever possible should be carried out to comply with BS3998 , there is not case of only getting the minimum clearence because of public outcry in conservation or tpo areas its in fact down to the company carrying out the work , the intrest of the serveyor and the skill of the climber carrying out the work to the tree . BUT some companies are not willing to spend time working to BS3998 and that is when you get the L shaped trees etc but what also needs to be taken into account is the utiilittys act which in fact allowes a powerline clearence contractor to cut any tree that is touching powerlines and could result in electrocution or death to gain a 1 meter clearence without having to ask the LA's permision ,however this act IMO is obused and used far to easily . as for cutting one side off of a tree to gain a clearence is still not acceptable and the company that has done the work could be liable should the tree or assosiated trees fail or become dangerous . Basicly there is still no need for trees to be butchered for the sake of powerlines . as for the railway im not at all sure how there rules and regs work