Hi Renocerous
These allow you to place a 'bump' or 'dip' in the track, for the train motion (sway) system in Trainz.
A good example of this would be to place one where you want to have a 'mud pit' (where the ballast has mixed with dirt, and turned the whole lot to mud, making the track drop down under the train). Another example might be where you have raised dirty to cover the track a little (ie a dirt road crossing the track), and want the train to 'bump' around a little when it passes over this section.
I'm sure others have found uses for these as well. It should be noted that junctions (turnouts/points) will automatically cause the train to bump/sway, as will level crossing objects.
Regards
Zec