The majority of the population of London once resided in and around the city core. The lack of a good public transportation system caused these central streets to be congested and over crowded.
The first underground line created; connected the railway termini, relieved congestion and enabled people to travel into the central core.
By this time the tube was expanding outward to service masses of people living further from the central core of London.
The most important development to the metropolitan line was the northward expansion into Middlesex country side which stimulated growth in the new suburbs.
Growth in suburbs continued to expand, hence the underground was now growing to meet the demand. It was becoming possible to commute to and from the city much more frequently.
After the war concluded, life resumed in London. People needed access to amenities which remained concentrated in the downtown core. Suburbs were also more widespread, making commuting even more necessary.
The immense effect of the expansion of this period can be seen here. The underground has been developed to take its completed form.
The suburban area and therefore population has expanded far away from the city center due to the tube. This has helped to relieve congestion in the city by enabling people to live in suburbs and still commute easily into the city. The downtown core remains constant while the tubes grow to serve the spreading suburban population.