In message , Huge
writes

Bicycles are not allowed on the _underground_ sections
of 'deep tube' lines (Northern, Central, Piccadilly, Victoria, Jubilee &
Bakerloo) at any time but are allowed off-peak on the Metropolitan,
Hammersmith & City, District

All of those I understand. The trains are, well, train sized.

and the Circle line.

This I do not understand.

The Circle line uses exactly the same type of train (C Stock) as the
Hammersmith & City and some District services. Also, the Circle shares
pretty much all of its track and all its stations, with at least one of
Met, District and H&C.
--
Spyke
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