Original: 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 their being "genetically modified" with General Motors equipment.
Simplified: They are known as "bodysnatchers", or "Zombies" to enthusiasts, because the shell (body) of the class 47 has been stripped, rewired and re-engined.