On the freights passing by me here in Witham on the East Anglia main line to and from Felixstowe, I see a mix of diesel and electric locos. (Aside: Electrics are sometimes doubled-up but never diesels.) I'm a great fan of it isn't broken don't fix it, but these locos must be about 50 years old. Is there an comfortable surplus of stock? Are they going to reach the end of their life and be replaced by new electrics? Or are we witnessing the twilight of a traction era?