How safe is the tube at about 11pm?

Speaking as a semi-regular tourist, just keep your wits about you as you
would in any major city and you'll be fine. Know where you're going (or fake
it well), keep your valuables close at hand, and be aware of what's going on
around you without being paranoid.

When I was lost a couple of times in the city, I'd stop for some tea or hot
chocolate, and use that time to get my bearings. A company called Pop-Out
Maps makes a good credit-card-sized one that has the central city in some
detail along with a basic Tube map for that area. It folds up quickly and is
discreet, almost spy-like.

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