def filter_valid_transit(df_in, hour_in):
    """Filters master dataframe for VEH_IDs valid for specified hour
    Inputs: df_in, a pandas dataframe containing joined transit_lines and
            transit_types excel worksheets.
            hour_in, an emme hour code as user arg.
    Returns: transit_list, a list of valid VEH_IDs per specified hour
    """
    df_in = df_in[df_in[hour_in].notnull()]
    transit_list = df_in['VEH_ID'].tolist()

    return transit_list