They are known as `` bodysnatchers '' , or `` Zombies '' to enthusiasts , because the shell ( body ) of the class 47 has been stripped , rewired and re-engined . They are known as `` bodysnatchers '' , or `` Zombies '' to enthusiasts , by virtue of the fact that the shell ( body ) of the Class 47 has been stripped , rewired and re-engined and as `` GM `` s due to them being `` genetically modified '' with General Motors equipment .