title: Open Street Map using OSMNX: how to retrieve the Hannover subway network?
body: <pre><code>import osmnx as ox
  ox.__version__ # '0.13.0'
  </code></pre>
  
  <p>I would like to show the <a href="https://www.openstreetmap.org/relation/4447844#map=10/52.2495/9.5736&amp;layers=T" rel="nofollow noreferrer">subway in Hannover</a> as known in the <a href="http://osm-subway.maps.me/germany.html" rel="nofollow noreferrer">German subway OSM data</a> on a map using the great OSMNX module. But unlike the <a href="https://github.com/gboeing/osmnx-examples/blob/master/notebooks/00-osmnx-features-demo.ipynb" rel="nofollow noreferrer">New York example</a> no results are returned for:</p>
  
  <pre><code>G = ox.graph_from_place('Hannover, Germany',
                          retain_all=False, truncate_by_edge=True, simplify=True,
                          network_type='none', custom_filter='["railway"~"subway"]')
  
  # EmptyOverpassResponse: There are no data elements in the response JSON
  </code></pre>
  
  <p>I do get results for other similar queries using 'Hannover, Germany' as region. I also do not get subway results for Paris or London. And I do not get results for similar queries like custom_filter='["railway"~"tram"]' or '["railway"~"s-bahn"]' or '["network"~"metro"]'. </p>
  
  <p>Also, if I use the infrastructure keyword argument to select "railway", an extensive gdf is returned:</p>
  
  <pre><code>G = ox.graph_from_place('Hannover, Germany', retain_all=False, truncate_by_edge=True, simplify=True,
                          network_type='none', infrastructure='way["railway"]')
  gdfox = ox.graph_to_gdfs(G, nodes=False, edges=True, node_geometry=True, fill_edge_geometry=True)
  gdfox.shape # (4422, 14)
  </code></pre>
  
  <p>But I cannot identify the subway using the columns returned?:</p>
  
  <pre><code>['u', 'v', 'key', 'osmid', 'service', 'oneway', 'length',
     'geometry', 'name', 'maxspeed', 'ref', 'bridge', 'tunnel',
     'access']
  </code></pre>
  
  <p>What I also find strange is that there are only 2 LINESTRINGS returned if I (try to) retrieve all railways using the custom_filter:</p>
  
  <pre><code>G = ox.graph_from_place('Hannover, Germany', retain_all=False, truncate_by_edge=True,
                          simplify=True, network_type=None, custom_filter='["railway"~""]')
  gdfox = ox.graph_to_gdfs(G, nodes=False, edges=True, node_geometry=True, fill_edge_geometry=True)
  gdfox.shape # (2, 10) # returns only 2 LINESTRINGS: Altenbekener Damm
  </code></pre>