Unit 2: HS2: An example of government expenditure with a multiplier effect?

The biggest single construction project for years according to the BBC

http://www.bbc.co.uk/news/uk-16482412 video

....with a multiplier effect, says this article

http://www.bbc.co.uk/news/business-16467903

a) What is the size of the multiplier efect suggested by the article?
b) Explain how this multilier effect will come about