This paper and its companion describe an experimental study of some aspects of upheaval buckling of buried pipelines by means of a small-scale model apparatus. Several theories of upheaval buckling, in which the foundation is modelled as a rigid base, are reviewed. Then a theory of railway-track buckling due to Tvergaard and Needleman is described: key features are the modelling of the soil by a nonlinear force-displacement relationship, and buckling by growth and localization of a periodic initial mode. An alternative, approximate investigation of the same situation leads to a simple formula for the axial load at which the localization of buckling occurs. This formula involves the flexural rigidity of the pipe, the amplitude of an initial periodic imperfection and the plateau soil resistance: the detailed form of the soil-resistance as a function of displacement appears to be immaterial. Experimental observations are presented on the growth of both horizontal and vertical displacement in the pipe as the axial load is increased; and it is shown that the observed buckling loads agree satisfactorily with the theoretical analysis.