This paper is concerned with an investigation of the detection and estimation of cracks occurring in concrete bridges constructed by Indian Railways. The dynamic behaviour of cracks is modelled by linear viscous dampers at the locations where cracks occur, and the locations are estimated using the least-squares method based on measurable input and output data sequences. The selection of the measurement locations is investigated by numerical simulation, and it is found that the selection of the extremum, or its vicinity, of mode functions characterizing the transverse deflection of the bridges much improves the crack location estimation performance.