There are 600 but that includes interchanges of any type: tube, train, bus, bike etc.
See the related link for further information.
How many feet underground is the Circle Line in London, UK
The number of interchanges is unlimited. Usually 50-60 interchanges are performed, but sometimes well over 100 interchanges per match have been performed.
It is really very fast, compared to street transport like buses, it is pretty cheap, and you can do as many interchanges you want with one ticket (until it expires), unlike with buses where you need a new ticket every time you get a new bus. It also has an awesome history.
There are many rivers that have been pushed underground in London.
London Underground was created in 1863.
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Apply to London Underground Ltd.
The London Underground is the largest in Europe and the oldest in the world.
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In London Underground (tube) stations.