Abstract: A rail sensor unit (10) for attachment to a rail (12) of a rail track, and for sensing by attachment to the rail, acoustic signals and vibrations in the rail. The sensor unit (10) comprises a housing body (20) made in one piece, having a contoured sensing wall portion (22), and an interior compartment (24). The contour is tailored for fitting against a head, web or foot of a rail. At least one piezo-electric transducer (42) within the housing body (20) is coupled to the sensing wall portion (22) for sensing acoustic signals. The housing body (20) efficiently provides substantially all of the contact surfaces form-fitting to the rail. Electronic circuitry (52) in the housing has a controllable dynamic range configuration for both weak signal detection and strong signal detection. The electronic circuitry and electromagnetic shielding protection (48a, 50a) are mounted on a rigid-flex printed circuit substrate (46) folded in the interior compartment (24).