This unusual puzzle by G.A. Roberts appeared in the January 1941 issue of Eureka, the journal of recreational mathematics published at Cambridge University. It concerns the Piccadilly Circus station of the London Underground, which lies on the Piccadilly line between Green Park and Leicester Square and on the Bakerloo line between Charing Cross and Oxford Circus.
At a given time there are on the platform, escalators and subways, and in the trains, 128 people, all of whom travel by train, and none of whom return immediately by the way they have come.
Those who have come via Leicester Square are equal in number to those who are about to travel via Leicester Square.
The number of people who arrived by Bakerloo Line is equal to the number who intend to leave by the Piccadilly Line.
The number of people who are travelling from the street to stations on the Piccadilly Line is equal to six-thirteenths of the number who change from the Piccadilly Line to the Bakerloo.
The number who arrive from Green Park and then change to the Bakerloo is equal to the number who are about to travel via Green Park.
The number who are travelling from the street to the Bakerloo is equal to four times the number who arrive in Piccadilly trains but do not use the Bakerloo Line, and of these, twice as many come from Green Park as from Leicester Square.
By how many does the number of people who use the Bakerloo Line exceed that of those who do not?