The implementation of new high-speed transport technologies re-shapes the demand balance between transport modes and rebound effects may occur. In this paper first a definition of environmental rebound effects of high-speed transport is presented and various cases are discussed. Second, a method is developed to determine and quantify the environmental rebound effects employing life cycle assessment. The method is illustrated in a case study by investigating the greenhouse gas emissions of a frequently discussed future underground maglev train system for Switzerland.