OECD/ITF: JTRC Discussion Papers

Environmental aspects of inter-city passenger transport
Per KAGESON, Nature Associates, Stockholm, Sweden
Discussion Paper No 2009-28, December 2009

The 25% increase is nothing he shows, it is something he assumes in making the calculations, based on an Norwegian HSR-line.

And in conclusion, I'll cite Kågeson, p.25:

The conclusion of this paper is that investment in high speed rail is under most circumstances likely to reduce greenhouse gases from traffic compared to a situation when the line was not
built. The reduction, though, is small and it may take decades for it to compensate for the emissions caused by construction. However, where capacity restraints and large transport volumes justify investment in high speed rail this will not cause overall emissions to rise.

In cases where anticipated journey volumes are low it is not only difficult to justify the investment in economical terms, but it may also be hard to defend the project from an environmental point of view as it will take too long for traffic to offset the emissions caused by building the line. Under such circumstances it may be better to upgrade an existing line to accommodate for somewhat higher speeds as this would minimize emissions from construction and cut emissions from train traffic compared to high speed rail.