Hi and welcome to Trainz,
There are more ways to do it, but this is probably the most easy one:
On the track tab, go to the "advanced" options. At the bottom, enter the requested hight you want one part of your track to be at, then go to "Apply vertex hight", the button right of it. When you have set the hight of both ends of your track this way, go to "smooth sline hight", button in the top right corner of that advanced section. Now click on the track and "tada", the ground rises to just below your track.