I did hear that there was a Castle on Reading shed all polished up and with crew in case they needed to change at the halfway point, but I don't know if that was just the imagination of the dyed-in-the-wool GWR foreman at Taunton.
There were several other locos done to exhibition finish for the occasion; a Nine Elms 80xxx pulled the stock in from Clapham and was against the buffers at Waterloo as the funeral party passed by (this may feature on television clips), steam heating it until the train loco came on, and Oxford likewise prepared a loco to bring the stock back from Handborough.
Many of the train magazines of the time had cover photos and articles inside in their March 1965 editions. The Railway World cover photo, from down by the riverside at Richmond Bridge with the train crossing over and the old gentlemen on the riverside path watching it with their hats off was probably the most striking.