maputnik/src/config/tilesets.json

{
  "openmaptiles": {
    "type": "vector",
    "url": "https://free.tilehosting.com/data/v3.json?key={key}",
    "title": "OpenMapTiles"
  },
  "thunderforest_transport": {
    "type": "vector",
    "url": "https://tile.thunderforest.com/thunderforest.transport-v1.json?apikey={key}",
    "title": "Thunderforest Transport (heavy)"
  },
  "thunderforest_outdoors": {
    "type": "vector",
    "url": "https://tile.thunderforest.com/thunderforest.outdoors-v1.json?apikey={key}",
    "title": "Thunderforest Outdoors (heavy)"
  }
}