This article takes a market access approach to study the effect of a major transport development, the introduction of steam-powered transport, on urban population change for 415 towns in England and Wales between 1830 and 1911. The totality of the roads, inland waterways, coastal routes and railway networks recorded in available sources for these two dates were digitised with unprecedented accuracy, in order to build a multimodal transport network accordingly. Our baseline model produces the unexpected result that, on average, improved market access had a negative effect on urban population growth. More specifically, the elasticity of population change with respect to market access change was of approximately -0.25. We argue that this unexpected effect was due to the strong heterogeneous effects of market access according to initial town size. Indeed, our estimates show that while market access had a significantly positive effect on population growth in large towns, it had a significantly negative effect on small towns. We argue that this heterogeneous effect was due to the fact that improved transport networks reinforced agglomeration effects in large urban centres while diminished the dispersion effect that had previously protected small towns.