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Edit the extension files
That was very vague. You assume @Shawn1995 knows what to edit in the extension file. @Shawn 1995, in all the trains folders, there is file called 'extension.cfg', which can be opened in either Notepad or Notepad++. After opening the extension file, duplicate the file in to separate folders, for the different configurations you want. The R110A comes in a 10 car configuration, so you would delete lines in the code that correspond to configuration you want. (delete 2 cars for a 8 car set, delete 4 for a 6 car, etc.)