Features should be taggable as underground, at level, or elevated

[BrunoSalerno]

And this should be represented in the map

[Kalepom]

When imported from OSM, this info is set with the tunnel tag

[BrunoSalerno]

This could be a little bit more complex: Tracks could be at level ...-2, -1, 0, 1, 2... If level == 0, it means that it is at ground level; if it is = 1, it is elevated. If it is < 0, it's underground. This value should be used to set the rendering order of the lines (with an order bystatements in the feature_collection query).