def train_test_split(t, df):
    """
    Splits time series dataset into train, validation and test set with a 60-20-20 split.
    Targeted to be used in an apply function to a Series object.
    :param t: current timestep (int)
    :param df: pandas Dataframe containing the data
    :returns: subset categorization as string
    """
    if t < (len(df) * 0.6):
        return 'train'
    elif t < (len(df) * 0.8):
        return 'val'
    else:
        return 'test'