I hope your planning on going beyond what Carto does. Baremaps currently doesn't render highway=busway either, leading to awkward gaps in the map. It's an accepted tag supported in every renderer except Carto (look at OsmAnd or Organic Maps for instance), because the two or three active maintainers remaining (having lost a number of maintainers in the past two years) don't like the tag. Carto has pretty much stagnated despite its past achievements.

As the default renderer for openstreetmap.org, Carto is currently an embarrassment for OpenStreetMap.