无法使用MKDirectionsTransportType.transit获取路线

Question

以下代码允许用户点击路段控制器以选择首选的运输方式，并返回从起点到终点的方向。我能够成功获得步行和驾车的路线。

@objc func handleDirectionSegmentControl(_ sender: UISegmentedControl){
    //transport = MKDirectionsTransportType.automobile
    switch sender.selectedSegmentIndex {
    case 0:
        transport = MKDirectionsTransportType.walking
    case 1:
        transport = MKDirectionsTransportType.automobile
    case 2:
        transport = MKDirectionsTransportType.transit
    default:
        print("Invalid index selected in Segment Controller")
    }

func getDirections(){
    let request = createDirectionsRequest(from: location)
    let directions = MKDirections(request: request)
    resetMapView(withNew: directions)
    directions.calculate { [unowned self](response, error) in
        for route in response.routes {
            let steps = route.steps
            self._steps = steps    
            self.mapView.addOverlay
            self.mapView.setVisibleMapRect(route.polyline.boundingMapRect, animated: true)
        }
    }
}

func createDirectionsRequest(from coordinate: CLLocationCoordinate2D)-> MKDirections.Request {
    let destinationCoordinate       = getCenterLocation(for: mapView).coordinate
    let startingLocation            = MKPlacemark(coordinate: coordinate)
    let destination                 = MKPlacemark(coordinate: destinationCoordinate)
    let request                     = MKDirections.Request()
    request.source                  = MKMapItem(placemark: startingLocation)
    request.destination             = MKMapItem(placemark: destination)
    request.transportType           = transport   //From Segment Controller
    request.requestsAlternateRoutes = true
    return request
}

我正在使用纽约市的起点和终点，那里在起点/终点之间肯定有公共交通选项。在我的iPhone（带有Apple Maps）上，Apple Maps功能能够显示我的应用失败所在的相同开始/结束位置之间的大众运输选项。

由于成功获得了行车路线/步行路线，我想知道是否需要实现.transit的其他功能。