The travellers are the girders that connect the drawer to the moving component of the drawer slide. That is, they travel in and out with the drawer.

These were mounted so that their top edges were about 1/16" above the static part of the drawer slide - just enough to ensure that the drawer wouldn't drag.

All five carrier / slide / traveller assemblies are (at this point) screwed to the benchwork on the back side - the fronts are floating to allow tweaking of the angles, if necessary.