1. Goto c:\Users\YOUR_USER_NAME\Documents\My Games\TrainSimWorld3\Saved\Config\WindowsNoEditor\
2. find or create the file Input.ini
3. open the file and add all inside the lines
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[/script/engine.inputsettings]
AxisConfig=(AxisKeyName="MouseX",AxisProperties=(DeadZone=0.000000,Sensitivity=0.070000,Exponent=1.000000,bInvert=False))
AxisConfig=(AxisKeyName="MouseY",AxisProperties=(DeadZone=0.000000,Sensitivity=0.070000,Exponent=1.000000,bInvert=False))
AxisConfig=(AxisKeyName="MouseWheelAxis",AxisProperties=(DeadZone=0.000000,Sensitivity=0.050000,Exponent=1.000000,bInvert=False))

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save the file and make it READ ONLY
(right click on Input.ini -> Properties check the box Read-only -> Applt and OK)