In May 1964 it was booked to work the London Paddington to Taunton leg of Z48 on Saturday 9 May, where upon arrival at Westbury it was discovered that it had melted its firebars from the heat of its fire, blamed on the provision of unusually high-calorific-value, carefully screened coal which caused the fire to burn white-hot as the speed exceeded 90 mph. This special coal had been sourced in a well-meaning gesture by British Railways in the knowledge that the Castle would be deliberately pushed as close to 100 mph as possible on its last scheduled service. As there was no standby Castle at Westbury and the closest Castle was in Taunton GWR 6999 Capel Dewi Hall took its train onto Taunton where 7025 Sudeley Castle took over for the journey to Plymouth.