@classmethod
def to_route_short_names_as_string(cls, short_names, sep=', '):
    """
        :return a string representing all short names (e.g., good for a tooltip on a stop popup)
        """
    ret_val = None
    for s in short_names:
        rsn = s.get('route_short_name')
        if rsn:
            if ret_val is None:
                ret_val = rsn
            else:
                ret_val = '{}{}{}'.format(ret_val, sep, rsn)
    return ret_val
