r run reference. This supersedes the integer
<code>run_id</code>. For backwards compatibility and since most run references are
integers, this function will attempt to convert an the argument to a
character. Run references may be retrieved from the
<code><a href='departures.html'>departures</a></code> or <code><a href='runs_on_route.html'>runs_on_route</a></code> functions.</p></td>
    </tr>
    <tr>
      <th>route_type</th>
      <td><p>Optionally filter results by a route type. A route type can
be provi