Experimental investigations were conducted on four artificially reconstituted materials, representing typical track foundation materials. Fractions of sand (Leighton Buzzard), silt (HPF4) and clay (Ball clay) were combined to create the four materials with clay contents of 7%, 11%, 14% and 24% respectively. Dynamic finite element analyses were conducted to obtain typical stress paths in a track structure. Principal stresses and their rotation during cyclic train loading were studied to define the scope for the laboratory element testing and to provide realistic magnitudes of applied axial and torsional load for the stress path testing. A hollow cylinder apparatus has been development and equipped with high-accuracy instrumentation to study principal stress rotation (PSR) during cyclic loading. Pairs of samples were tested to compare their behaviour during cyclic undrained loading with and without PSR. Firstly, cyclic rotation of the principal stress directions results in a decreased resilient Young's modulus compared to cyclic loading without PSR. Secondly, cyclic PSR significantly increases the rate at which permanent axial strain accumulates compared to cyclic loading without PSR, and the effect becomes more significant as the clay content of the material decreases. During cyclic PSR, pore pressures are generated that can lead to excessive plastic deformation and ultimate shear failure. Conventional cyclic triaxial tests, which have largely determined the state-of-the-art pavement design methods, can not model the important effects of PSR on the long-term behaviour of track foundations.