It's more complicated to explain what to do, than it is to actually make the changes, but here goes:
First you need to know who built the route, I'm going to use RSC BSB route as an example.
The first thing you would do is open the "Steam\SteamApps\common\railworks\Assets\Kuju\Rail SimulatorUS\RailNetwork\TrackRules" folder. ****"Note: The track file you'll be looking for will always be located in a 'TrackRules' folder" ****
Next you will need to open the "SanB_Bars_ Mainline.bin" file with either RW_Tools or the serz.exe application. Once you have the "SanB_Bars_ Mainline.bin" file open, you need to search for the <DefaultRideQuality> section.
<DefaultRideQuality>
<Network-iTrackNetworkRideQuality-cPropertyValue>
<LineUnevenness d:type="sFloat32" d:alt_encoding="0000000000805640" d

recision="string">20</LineUnevenness>
</Network-iTrackNetworkRideQuality-cPropertyValue>
</DefaultRideQuality>
Change the value in the <LineUnevenness line between the greater than and less than symbols ">20<" to a higher value for more cab sway or a lesser value for less cab sway. Then save the file and try the route.