Between the two World Wars (1919 – 39), the responsibility for housing the working classes was accepted by the British state, and a million houses were built for them in sub-urban areas by local authorities. The city had gradually extended beyond the range where people could walk to work. Development of suburbs made new forms of mass transport very necessary. The London underground railway partially solved the housing crisis by carrying large masses of people to and from the city. Underground railway created huge ecological and environmental problem. The process of construction led to large scale destruction of forests and other natural features.