Most UK railway embankments were rapidly built in the Victorian era using soil excavated from a cutting nearby, loosely tipped to full height without systematic compaction. This resulted in material heterogeneity with irregular clay fill slopes. The clay fill could be characterised as relatively dense and overconsolidated clay clods in a soft and remoulded clay matrix mixed with foreign material. The clay fill embankments have deteriorated due to cycles of both train and environmental loading. Triaxial test results confirmed the inherent variability of intact Lias Clay fill regarding the undrained shear strength, small-strain stiffness, pore pressure response and soil structure. The shearing response was governed by the soft clay matrix. The cyclic threshold stress of the clay fill was determined by a normalised stress ratio at ๐‘ž๐‘๐‘ฆ๐‘/๐‘€๐‘โ€ฒ = 0.8 that was valid for all sample types of Lias Clay.The mechanical behaviour of typical old railway embankments under different traffic and track conditions was investigated using a validated, advanced constitutive model fully coupled with pore water interaction as saturated porous media. A mixed finite element was developed through ABAQUS, which can effectively replicate the transient behaviour of an elastoplastic material under dynamic loading and partially drained conditions regardless of loading frequency and soil permeability. The developed algorithm is numerically stable, which enables short- and long-term simulations on a range of geotechnical applications. Finite element analyses of track-embankment-ground system with validated parameters indicated complex transient and residual behaviours of stresses, pore pressure and deformations under passages of a single wheel, a whole train and 100 trains. The effects of vehicle geometry, axle load, rest period, track support system modulus, soil permeability and slope gradient on plastic slope deformations and residual pore pressure were carefully investigated and discussed.