Thanks for your help buzz456

. I figured that out last night when I followed an instruction to install a re-skin. I extracted DonnerPassAssets.ap dated 06/05/2014 @ 1,306,789 KB with WinZip 17.5 and replaced DP_manualjunction.TgPcDx in the "...\Assets\RSC\DonnerPass\RailNetwork\Junctions\textures" directory.
I deleted Blueprints.pak and selected all the files and folders in the DonnerPass folder, except "DonnerPassAssets.ap.Original", and created DonnerPassAssets.zip with normal compression @ 611,184 KB.
Then I moved the new .zip file from the RSC folder to the DonnerPass folder and renamed the .zip extension to .ap
I deleted all the extracted material to the recycle bin, just in case, even though I also backed up the extracted material to my "Local Backup" drive.
A test of the route in Train Simulator 2016 was a complete success including no reduction in frame rates.
