Accelerometers on a bridge can be used to extract dynamic properties such as mode shape which are known to be damage-sensitive. However, as bridges are often physically large, this can require a significant number of sensors. In this paper, the concept of re-deploying a small number of sensors is proposed, building up the mode shape from a series of field acceleration measurements. Frequency Domain Decomposition is used to find segments of mode shape in each measurement, using a different train as the source of excitation in each case. In numerical simulations of a simply supported beam, it is shown that a minimum of two accelerometers can be used to construct two points of a mode shape, the first mode being the one most sensitive to excitation. Mode shape amplitudes are calculated in segments which are then stitched together to obtain the global mode shape. In numerical simulations, failure of the support bearings is represented as an increase in rotational spring stiffnesses. This is shown to have a significant effect on the shape of the first mode. The concept was tested in a field trial, before and after the bridge was repaired. The measured mode shape, represented as an envelope of possible shapes, was seen to change significantly when the support system was rehabilitated, and new bearings installed.