{
  "width": 100,
  "height": 60,
  "background": "#f2efe9",
  "features": [
    {
      "name": "track-no-tracktype",
      "type": "linestring",
      "tags": { "feature": "highway_track", "tracktype": null },
      "layers": ["roads-fill"],
      "zoom": 15
    },
    {
      "name": "track-grade1",
      "type": "linestring",
      "tags": { "feature": "highway_track", "tracktype": "grade1" },
      "layers": ["roads-fill"],
      "zoom": 15
    },
    {
      "name": "track-grade2",
      "type": "linestring",
      "tags": { "feature": "highway_track", "tracktype": "grade2" },
      "layers": ["roads-fill"],
      "zoom": 15
    },
    {
      "name": "track-grade3",
      "type": "linestring",
      "tags": { "feature": "highway_track", "tracktype": "grade3" },
      "layers": ["roads-fill"],
      "zoom": 15
    },
    {
      "name": "track-grade4",
      "type": "linestring",
      "tags": { "feature": "highway_track", "tracktype": "grade4" },
      "layers": ["roads-fill"],
      "zoom": 15
    },
    {
      "name": "track-grade5",
      "type": "linestring",
      "tags": { "feature": "highway_track", "tracktype": "grade5" },
      "layers": ["roads-fill"],
      "zoom": 15
    }
  ],
  "extra_tags": ["access", "bicycle", "construction", "feature", "foot", "horse", "service"]
}

Add an example for tracktypes.
Add an example for tracktypes.
