China clay traffic to the Potteries has been fairly stable, in the form of a a weekly trainload delivery to Cliffe Vale (Stoke), which has been the pattern for many years now. Currently serves Cliffe Vale on Fridays.
The trainload clay slurry "silver bullets" from Burngullow to Irvine switched to imports (in 2008 according to a post I found) which continued using rail through the tunnel from Antwerp until it ceased early last year.
There were some less-than-trainload flows from Cornwall to Scotland and elsewhere, some were lost with the end of Speedlink and some survived into the EWS "Enterprise" era before dwindling to nothing.
Volumes sent by rail within Cornwall to Fowey for onward shipment seem to have gradually declined over the years, I think it's more to do with economics and foreign competition than running out of clay.
Loading points have gradually been reduced to Goonbarrow (for Fowey), with the Cliffe Vale traffic being loaded at Treviscoe and Par Harbour. I'm not sure if the places that have ceased rail loading over the years (such as Marsh Mills) actually closed or switched to road transport.