How do I know if I should ask her for her phone number?

There is this girl I've met in public transportation, we would look at each other briefly with talking to each other and then keep on our way to our respective jobs.

Last week, when we got out of the train station, it was raining and we were walking one beside the other (we both walk pretty fast, or at least, faster than the average person) and started talking about the rain.

After that we have seen each other sometimes and we talk but there is one thing: when we walk together (reminding is not every day but some days) we walk fast (as stated above) and therefore we talk for a short time before parting ways to our jobs, we have conversations, yes, but short, and I don't know if I should ask her for her number, maybe it is time or maybe it isn't, but I don't know.

Thanks for the guidance.

How do I know if I should ask her for her phone number?
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