Some users have found that the 64-bit version is not running as well as the 32-Bit version. The common cause of this appears to be that the 64-bit version is using the Integrated Graphics device rather than their discreet high-performance graphics device.
To resolve such an issue, you will need to tell Windows to use your discreet high-performance graphics device when you launch Train Simulator 64-bit.
To do so please open the Windows Settings menu within Windows 10 and select System and then Display.
You’ll need to scroll down to the bottom of this page and select Graphics Settings.
From here select Classic App from the drop-down list and then select Browse. A pop-up will now appear asking you to select a file.
You’ll need to navigate to your Train Simulator installation folder and select the file RailWorks64.exe and then click on Add. By default, the Train Simulator is installed to C:\Program Files (x86)\Steam\steamapps\common\RailWorks but if you've told Steam to install it to a different drive or folder, you'll be able to find that by opening Steam, select Game Library, then right-click on Train Simulator and select Properties. Next, click on the Local Files tab and click the button Browse Local Files - the installed folder location will be revealed in the Windows Explorer address bar.
You’ll now see RailWorks64 listed on the Graphics Settings page please click on this and select Options.
From the three options which appear, you’ll need to ensure High Performance is selected. Once selected press Save to confirm your choice.
Afterwards, you should now find that when launching the 64-Bit version of Train Simulator 2020 you get the full performance your system is able to provide.