A spike-driving machine for use in driving spikes into railroad ties and including a rail clamping apparatus for securing the machine to the rails while spikes are being driven into the rails. The rail clamping apparatus can employ only a single hydraulic cylinder and is constructed so as to firmly grip the rails even if the heads of the rails are substantially worn and also constructed so as to grip a variety of different sizes of rails whether or not those rails are new rails or substantially worn.