The guard is here responding to a request from the driver (through a series of coded blasts on the whistle). He is waving to inform the driver that the last coach of the long train has cleared a bridge on which there is a strict speed restriction. The tightly curved track means that he has to lean a long way out to be seen by the driver - who then whistles again to indicate that he has seen the guards' signal. Imagine the Health and Safety Executive allowing this in the UK!?!