The class 47 (or Brush Type 4). Much maligned as many railway enthusiasts thought they were boring, but they built 512 of them, they could do anything from slow speed freight to high speed passenger, were reliable, easy to maintain, and lasted in mainline service for over 40 years.
And I'm part owner of the loco featured in this article...
BBC News - Railway locomotive celebrates 30 years of service
The Gloucestershire Warwickshire Steam Railway says the milestone is down to volunteers' hard work.
www.bbc.com