It is a relative figure from a general uplift.
We should look at the experience of countries that have high-speed lines, such as France, Spain and Germany. The most direct comparator is the line between Lyon and Paris. When the Transport Committee went there in 2011, it found, and was told by the director-general of SNCF, that both cities had benefited from it. All the economic benefit had not been sucked out of Lyon and into Paris; both ends had benefited. The same is true of the lines between Frankfurt and Cologne and between Lille and other parts of France. That does not just happen because the line is built, however; it happens if there is a strategy of the Government and the city governments to make sure there is benefit from that high-speed route. It relies on active involvement from local and city government to make sure all the benefit of that investment is captured.