Graham, this sounds like your backups folder is getting a bit stuffed with old files.
Open up the Launcher then settings.
Open up the Install tab.
Look at where the Trainz content is installed and jot that down on a piece of paper.
Close this and close the Launcher.
Open up Windows Explorer (aka File Explorer today).
Go to the folder you wrote down before.
Open up the Backups folder and delete the oldest dates.
Close Explorer and Empty the recycle bin.
You have now reclaimed a lot of space.
Unfortunately, Trainz content is now a lot bigger in addition to the backups and this doesn't help the matter, but at least clearing the old stuff helped.
I recommend defragmenting your hard disk afterwards, or run the trim function if it's an SSD. This will help with performance after removing the data.
John
Trainz User Since: 12-2003
Trainz User ID: 124863
TRS2019/Trainz-PLUS: 114800
TRS22 Beta tester