The double arrow symbol on its own painted in white, often used on the sides of rolling stock and most prominently on the "Big Logo Blue" livery for locomotives from 1967 to 1982.
History:
The Double Arrow is a railway symbol originating with British Railway's 3rd rebrand in 1965 to British Rail, being their third logo since inception.
The symbol was intended to represent British Rail and to be used on their rolling stock, but was and still is used more broadly as a symbol for railway stations.
The double arrow was used as the logo for national rail (symbolising national railway stations and tickets) after British Rail was privatised in 1995-97.