Modify presence of routes.network_id

Changed to Conditionally Forbidden

gtfs/spec/en/reference.md

@@ -214,7 +214,7 @@ Primary key (`route_id`)
 |  `route_sort_order` | Non-negative integer | Optional | Orders the routes in a way which is ideal for presentation to customers. Routes with smaller `route_sort_order` values should be displayed first. |
 |  `continuous_pickup` | Enum | Optional | Indicates that the rider can board the transit vehicle at any point along the vehicle’s travel path as described by `shapes.txt`, on every trip of the route. Valid options are: <br><br>`0` - Continuous stopping pickup. <br>`1` or empty - No continuous stopping pickup. <br>`2` - Must phone agency to arrange continuous stopping pickup. <br>`3` - Must coordinate with driver to arrange continuous stopping pickup.  <br><br>Values for `routes.continuous_pickup` may be overridden by defining values in `stop_times.continuous_pickup` for specific `stop_time`s along the route. |
 |  `continuous_drop_off` | Enum | Optional | Indicates that the rider can alight from the transit vehicle at any point along the vehicle’s travel path as described by `shapes.txt`, on every trip of the route. Valid options are: <br><br>`0` - Continuous stopping drop off. <br>`1` or empty - No continuous stopping drop off. <br>`2` - Must phone agency to arrange continuous stopping drop off. <br>`3` - Must coordinate with driver to arrange continuous stopping drop off. <br><br>Values for `routes.continuous_drop_off` may be overridden by defining values in `stop_times.continuous_drop_off` for specific `stop_time`s along the route. |
-| `network_id` | ID | Optional | Identifies a group of routes. Multiple rows in [routes.txt](#routestxt) may have the same `network_id`.| 
+| `network_id` | ID | **Conditionally Forbidden** | Identifies a group of routes. Multiple rows in [routes.txt](#routestxt) may have the same `network_id`.<br><br>Conditionally Forbidden:<br>- **Forbidden** if `route_networks.txt` file exists.<br>- Optional otherwise. 
 
 ### trips.txt