Elevation offsets are used in conjunction with the Draping settings on the Source tab. You can move aril lines 5 meters below ground level by using an elevation offset of -5. Then use the draping setting to place rails under the terrain (in tunnels). Use similar adjustments to place rail lines above the terrain (on bridges) or directly on the terrain. The automatic styling of the rail lines reflects these choices.
You can also use an expression to map a property from the data store to one of these attributes. See Use Expressions.