So what happens if you use your recent Win10 installation to control your layout instead of the XP version. Does the issue go away. If it does, then that would prove it is definitely an issue with the 1.72 RailMaster installation on the old XP laptop as you suspect.
If the diagnostic test I have suggested above proves beyond doubt that the issue is with the XP Laptop RM installation, then I suggest you manually update the XP to 1.73 and test again.
Note that RailMaster will not automatically update from 1.72 to 1.73.
1.73 needs to be manually downloaded from the link at the top of the RailMaster forum and manually installed over the top of the 1.72 version. Remember to run the 1.73 installer by 'right clicking' the downloaded installer file and choosing 'Run as administrator'.
Although RM was originally written for XP, it has been modified extensively to get it working with initially W7, then W8 and now W10. Re-installation using this latest 1.73 version may possibly fix it as your symptoms do sound like a possible file corruption issue of some kind.
The 'bite the bullet' solution may require you to delete your locos from your loco roster list and re-add them again from scratch. At least this 'last resort' solution can be tested by just initially trying it out with a single loco to start with. Historically, deleting the locos and re-adding them has been proven to fix other throttle slider related issues. But in the past, users have usually only reported 'throttle slider' issues on one loco at a time, not the whole fleet.