Running empty trains (ecs) never seemed to be cost effective to me. Having worked from Weymouth to Westbury 37213 is seen returning stock empty back in order to work the 16:58 Weymouth – Cardiff on 23rd July 1990.
Lightly loaded, having not waited for the boat connection at Fishguard Harbour 37218 waits time at Llanelli bound for Paddington on 22nd September 1984.
Don't think anyone would ever believe that in 2015 no-heat 37s would be working passenger services on a regular basis. 37218 (with 37402 on the rear) waits to depart Barrow-in-Furness with the 17:32 to Carlisle on 29th June 2015.