["Journal of Agricultural Economics, EarlyView. ", "\nABSTRACT\nImportant developments in infrastructure and institutions characterized 19th‐Century wheat markets in Great Britain. Among these developments was the construction of the national rail system which enabled cheaper and more efficient transport of grain and other bulky cargoes between inland towns and ports. We consider the degree to which wheat markets in twelve British cities were spatially integrated over the nineteenth century by applying conventional threshold models and semi‐nonparametric (generalized additive) models of spatial price linkages. Although results concerning market integration are qualitatively similar, the semi‐nonparametric models capture important nonlinearities that may be missed by conventional threshold models. Both approaches support a high degree of integration among the spatially distinct markets. We then consider two sub‐samples from 1800–1840 and 1870–1913 to capture price transmission before and after the repeal of the Corn Laws, the development of the railway system, the improvement of steam engines for ocean transport, and the introduction of the telegraph system. The nonlinear models suggest faster equilibration in response to large shocks and tighter transaction cost bands in the second period.\n"]