I have a 1990 Williams GG1. Beautiful train! The revering unit works well. Now I would like to run the train in one direction only. I do see any switch to activate this on the outside of the train. Is this switch on the inside?

There is no direction lock switch on Williams, or early Williams by Bachmann, locomotives.  To add a switch you can replace the original reverse board with the newer version that has provision for adding a direction lock switch.

New board: https://estore.bachmanntrains.com/index.php?main_page=product_info&cPath=77_222&products_id=10587

Installation instructions: https://www.bachmanntrains.com/home-usa/dwg/dwgs/WilliamsReverseBoardInst(later).pdf

Len