Cyanide ampoule 

Women workers of the Plessey firm work in London Underground railway tunnels during the war.  At the outbreak of war, the London Underground was in the process of extension and modification.  This involved the construction of new tunnels and the abandonment of some old stretches.  The resultant "unused" stretches of tunnel were intensively exploited for the war effort - as shelters, as secure storage and - as here - as underground factories.  

During the war, the Plessey company supplied the government with a range of goods, from electronics to shell cases.  These ladies are probably working in a five-mile section of (new) Underground tunnel on the Central line which had not been brought into service by the outbreak of war.  Other abandoned/uncommissioned tunnels in London, as well as tunnels and caves of all sorts across the UK, were employed in "unconventional use" in the interests of the war effort.  Best regards, JR.