Why must use if for this sentence?

[paul harris]

English Grammar Tests, Elementary Level

ESL/EFL Test #70 "Chit Chat: In the train", question 1

Mrs Ashton: Excuse me but do you know ......... this train is going to London?

(a) since
(b) if
(c) how
(d) whenever

English Grammar Tests, Elementary Level

ESL/EFL Test #70 "Chit Chat: In the train", answer 1

Mrs Ashton: Excuse me but do you know if this train is going to London?

Correct answer: (b) if
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why must use if for this sentence?

paul harris

[Alan]

Hi,

This is an open question to which Mrs Ashton doesn't know the answer and that is why she uses if because the answer will either be yes or no.

Alan
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English as a Second Language
You can read my ESL story Phrasal Verbs/run

[Hani]

I think this sentence should be: "Do you know when this train is going to London?"
Am I right?

[Yankee]

Hi Hani

The correct answer is if. The word when was not an option in the test.

It would also be better to use the simple present tense to talk about scheduled times for trains.