The official competition is being run by Crossrail, and is open to people aged between three and 15. I am using this blog to open an unofficial version of the competition to all my followers.
People in the age range to qualify for the proper competition can visit the site here (the competition is open until Sunday). For those not eligible for the official competition you can use this post to present your ideas, either in pictures if you have the skill and inclination, or in words, or as a combination of both. If you put your thoughts into a post of your own, include the link in a comment and I am sufficiently impressed I will reblog.