I have no idea what happened but the railworks64.exe disappeared from my railworks directory. This then caused steam to try to reinstalled the game and said missing 64bit version.
Reading all the threads concerning problems with TS after the Classic update (which did not change anything except the version number in the RailWorks64.exe), the OOMs and missing exe people are reporting are due to your AntiVirus software messing with Train Simulator. pennsy_railfan: Your Railwork64.exe is most likely in your AV software's quarantine folder. Make an exception for it. If you do a file verification on Steam, you'll get it back but unless your AV software is updated the same will happen again. To everyone else I highly recommend getting rid of payware Antivirus software and switch to the built-in Windows Defender, which is according to many experts, the best protection. Especially stay away from Kaspersky.
I stopped using payware antivirus software years ago and switch to Sophos Home free version and never had any issues with anything.
I did a complete reinstallation when TS updated to classic. I am using Eset Internet Security and F-Secure Freedome VPN. And have no problem with Trainsimulator at all.