This paper considers the scope for wider economic impacts of agglomeration-based effects in the context of high-speed rail. The paper provides new evidence concerning the distance decay of long-distance travel and on the basis of such estimates assesses the potential order of magnitude of agglomeration benefits from improvements to high-speed rail transport in Great Britain. A new methodology assumed constancy of trip decay with respect to equal average travel times but variability with respect to distance. The expected outcome was that improvement in travel times would reduce the extent to which travelers perceived distance as an obstacle to interaction. Although urban economic theory did not preclude the existence of agglomeration benefits across interregional distances, the empirical evidence suggested that the order of magnitude of agglomeration benefits corresponding to 25% and 50% increases in travel speeds was small.