What's the quickest way to get to the other underground lines from the northern line?
Is it to use the lifts or to leave the line via the new ticket level and walk above the ground to the old ticket hall
You'll need to be a bit more specific in asking which other line you want to get to. But the new ticket hall is the right answer to very few questions.
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For every other line you’re better off using the escalators at the south end of the Northern line platforms. These escalators take you to Piccadilly line level whereby you can turn right for a staircase to the Vic. For the Sub-surface lines you go left off the escalators and then back on yourself
for another bank of escalators to the Tube ticket hall, from which you can find a passageway to the Sub-surface.