The GSWR crest lasted three quarters of a century. The GNR crest lasted from 1876 to 1958. The GSR crest, 20 years. BCDR - longer. Flying snail - almost 20 years. UTA crests - about 10 years each. NIR logo as was - in various forms 1967-1996 or so. Broken wheel - 1962-87. IE Set of points - 1987 - maybe early 90s, getting shorter. Three-pin-plug - over 15 years. Recent green wiggly thing - not long.
So we are going to copy insurance firms and English "TOC"'s - i.e. these pixie outfits like the Grand Great Western of Cotswold and Ayrshire Train Station Company**, and change logo at huge expense to taxpayers every fortnight!!
(** who own three railcars, sorry; DMUs, and each one has a different livery apart from the garish yelllow ends!)
I'll just finish my tea and go. The snow's started.