A striking shot of Warship D831 Monarch heading out from the Western portal of Box Tunnel , bound for Bristol on an express from Paddington , construction of the tunnel having been started by Mr . Brunel in 1838 when his line from Paddington to Temple Meads heralded the start of through services between the Capital and the Westcountry. At the Eastern end of Box tunnel there still exists today a second and smaller entrance , hardly visible to today's rail travellers with a siding leading into it from the main line and which in 1936 came into use as the entrance to a munitions store for the War Office with underground quarries providing storage space for 250,000 tons of high explosives with some 1300 employees engaged there from July of 1938 . How much still exists today within the tunnel is not known but doubtless there are still reminders of its wartime role.