Many old railway embankments were originally formed from loose dumped clay fill on which ash fill was subsequently placed to maintain the track level. These have required considerable maintenance, primarily because of embankment movements. They are mostly covered by trees, and tree roots are present in both fills. As part of a London Underground Limited programme of stabilisation works in the 1990s, two embankments were instrumented to investigate the mechanisms and causes of movement. Lateral deformations, settlements and pore pressures were measured. This paper describes the instrumentation and monitoring techniques that were adopted and presents the findings from the study. It was found that non-recoverable seasonal movements occur in both the ash fill and the clay fill. The former occur in dry weather, particularly in the slopes of the embankments crests, due to ash particle mobility under train loading when the ash is dry. Clay fill deformations are exacerbated by the presence of tree roots. Movements correlate well with climate, as quantified by the soil moisture deficit determined from meteorological data. Establishing the mechanisms of movement within these ash–clay fill embankments helped to guide the design of stabilising measures.