This may be related to the locomotive you are using. If you are using Jointed Rail locomotives or those that use JR's locomotive libraries, then there are options under the 3-bar Trainz settings in the program.That was not what I was referring to. In the past when driving in-cab mode and the engine starts to sway the horizon would not. Now the horizon will sway as the cab sways. To see what I mean create a new 6 baseboard route and change the track condition to 25 and add a speed limit of 70mph then give the driver the Drive command and go to in-cab view. As the train speeds up watch the horizon. At about 45 the horizon should start to rock back and forth.
Click on the Jointed Rail Settings library v 0.5
There's a slider located at the top of the window on the right:
Cab Sway Amount: 30 %
30% is the default. You can add or reduce the amount of sway you want.
This is independent of the track conditions. While they appear to work together sometimes, they are completely independent.