The Great Train Robbery on the Romney, Hythe and Dymchuch

A gang of criminals, led by Derek Guyler and Clive Dunn, restage the Great Train Robbery on the Romney, Hythe and Dymchuch Railway.

This is a sketch from Michael Bentine's show It's a Square World, which ran from 1960 to 1964.

The BFI site makes it sound something of a lost gem:

It's a Square World employed location filming, ingenious special effects and elaborate models to realise the more extreme visions from the imaginations of Bentine and his co-writer John Law. Ranging from gentle satire to surreal slapstick, the fast-moving agenda was anchored by Bentine, usually appearing as a hapless authority figure trying manfully to remain calm in the face of a conveyor belt of unlikely eccentrics and lunatic situations. 

Recurring characters included Oil Sheiks speaking in a cod-Arabic language of Bentine's own invention; feuding magicians; inept Russian spies and incident-prone Egyptologists. Themes included the military, the United Nations, uncharted lands and the BBC itself. 

Skits were often linked by Bentine as a newsreader introducing stories and introducing assorted correspondents in the field. Memorable moments included the BBC Centre portrayed as a POW camp that held the Corporation's creative staff to prevent them defecting to ITV; the Slabodian mountaineering team's attempt to scale the Woolwich gasometer; the Triffid invasion of the BBC; and the sinking of the House of Commons by a Chinese junk. 

Many of the themes and character types were echoed later by the Monty Python team, especially Bentine's linking newsreader and his preoccupations with the military and bizarre officialdom (a Square World sketch featuring the Ministry of Holes predates the Pythons' Ministry of Silly Walks).