(fine-tune-wav2vec2-english.md) What does 'remove_columns=timit.column_names["train"]' do?

[epona7471]

def extract_all_chars(batch):
all_text = " ".join(batch["text"])
vocab = list(set(all_text))
return {"vocab" : [vocab], "all_text" : [all_text]}

vocabs = timit.map(extract_all_chars, batched = True, batch_size=-1, keep_in_memory=True, remove_columns=timit.column_names["train"])

I can't get the meaning of 'remove_columns=timit.column_names["train"]'.
Looks like it is removing training dataset, but output var 'vocabs' also have both 'train' and 'data' keys.
Result lookssame with 'remove_columns=timit.column_names["test"]'.

Newbie looking for a help.

Thank you