"This train is a Northern Line service calling at all stations to Morden" the platform supervisor at Embankment called over the tannoy system on the southbound platform as the last passengers boarded the service just moments before the doors were due to close.

"Stand clear of the doors please!" the supervisor added, emphasising the please as much as possible but still failing to deter the inevitable last minute passenger who leapt aboard literally as the doors were closing.

After a moment, the brakes were released and with a whirr of electrical motors, the six car train of 1995 type Northern Line tube stock accelerated quickly away into the far tunnel portal and vanished, leaving behind a turbulence of wind and a rapidly fading rumbling echo.

The platform supervisor decided to take a walk up the length of the platform and check everything was alright. Nothing appeared to be out of place and all was quiet bar the distant echo of footsteps and the arrival and departing of trains elsewhere in the station complex which was filtering through the various passageways and tunnels.

As seemed to be the standard case, the chocolate vending machines were marked out of order but apart from that, all seemed normal.  The platform supervisor was about to leave and cross over to the northbound platform when he caught out of the corner of eye, something flapping beneath a bench at the far end.

With some curiosity now raised, the supervisor walked back down the platform to the bench and bent down to look at the object that had attracted his attention, a simple canvas bag, opened and with its contents clearly visible.

"Kerry" the platform supervisor called into his station radio "Give the Transport Division lads a call will you, I think we've got something down here on the Northern southbound."