As the second world war broke out, the movement of supplies became paramount to winning the war effort. The control of the railways was passed to the ‘Railway Executive Committee’ who tried to put people off travel with this posters. The National Railway Museum put it better than I cold so below is some text from them:
Stay at home
Once the Railway Executive Committee took control of the network, hoardings were immediately cleared of most advertising material relating to leisure travel. The holiday resorts on the southern coast were effectively closed because of the threat of invasion, and the railway industry geared up for its essential role as part of the war machine moving men and material. Station names were painted out to confuse the enemy in the event of invasion and there was a universal black out removing all lighting which might attract bombers on night air raids.
By November 1939, the British railway network was seriously preparing for war. It was moving extra food supplies, equipment and troops essential for the logistics of conflict. The materials to build the new war factories, the raw materials to make the munitions of war and the men and women who fashioned them all had to be carried on the railways.
From June 1940, East Coast shipping was heavily cut back, and much of this freight was transferred onto the East Coast Main Line. On some sections, traffic rose by 500 percent.
The public were urged to spend their holidays at home, as the running of additional trains during the summer months and bank holidays were now a thing of the past. Travel for pleasure was discouraged throughout the war, as the railways were now the lifeline of both the military and public services.
http://www.nrm.org.uk/AboutUs/PressOffice/PressReleases/2009/May/thera