Interesting, according to the Office of National Statistics (Travel Trends International Passenger survey 2005 published Nov. 2006
http://http://www.statistics.gov.uk/pdfdir/ttrends1106.pdf ),
74% of all foreign visitors arrive by air, with a paltry 11% coming via the chunnel ( a combined figure for Eurostar as well as car carriage!), which figure was still eclipsed by 16% arriving by sea
Americans form the most valuable group economically (despite being less numerous that Europeans, they spend almost ten times as much per capita - presumably none of it on Eurostar tickets as there isn't a branch line to New York).
Clearly, the vast majority of Eurpoeans don't elect to travel by train (something that 20 minutes off the journey time from Potsdam/Barcelona by train ain't going to change) and the economic effect of european train travellers is minimal.
No doubt some leger de main could be exercised to show a return on this "
investment" fdirectly related to tourism (providing enough skewed questionnaires are issued) but it'll be closer to the date when hell freezes over rather than a rational ROI period.
On interesting nugget is a graph that shows that outside Lunnun, the most visited city is Edinburgh, followed closely by Glasgow, neither of which are served by Eurostar.