[professora]

Hi!

could you tell me which is the correct way to ask the time of departure, a.s.a.p.

Is it: What time does the train leave to London?
or: What time does the train to London leave?

Thanks!

[mesmark]

Both probably would be fine in the course of conversation but the second is more natural.
What time does the train to London leave?

leave to (place) is a little awkward in my opinion. We generally say leave for 'place'

What time does the train leave for London?

These two ways show up at about the same frequency:
What time does the train to London leave?
What time does the train leave for London?