A rail clamp comprising two clamp elements (30a, 30b) securable together in clamping arrangement about a railway rail, wherein at least one of the clamp elements (30a, 30b) can include sensor retaining means to hold a sensor to make measurements relating to the rail. A first of said two clamp elements (30b) has a clamp head to engage a fish plate (14b) or a web-side of a railway rail. A clamp foot (34b) engages the underside of a railway rail foot (13). A second of said two clamp elements (30a) having a clamp head to engage the opposite fish plate or web-side of a railway rail and a clamp foot (34a) to engage the underside of the clamp foot (34b) of the first clamp element (34b). The first and second clamp elements (30a, 30b) are secured together by a connector (35), optionally a bolt, to a railway rail.