A London Underground boss has revealed fascinating little known facts about the Tube.
Head of Network Delivery for Transport for London Richard Jones divulged trade secrets on topics ranging from the causes of signal failures to the fastest trains on the network in a new interview with MailOnline.
The London Underground, which opened in 1863, is the world's oldest underground railway network.
The Tube handles up to five million passenger journeys per day, and has 11 lines serving 270 stations.
Here are six things you may now know about the world-famous transport system:
What causes a 'signal failure'?
"The term 'failure' to describe a signal showing red is engineering terminology and could be described as a little misleading to the layperson," said Mr Jones.
"The signal itself may not actually have failed but the signal light remains red because something is making it unsafe for the signal to be turned to green and allow a train to proceed."
Reasons why the signal light remains red can include a cable fault or litter causing a shortcut in an electrical circuit.
Which line is the most reliable in terms of signalling?
The line with the fewest signal failures is the Waterloo & City line, according to Mr Jones.
He added: "However, it would be incorrect to call other lines, which are longer, run more trains and have more complex operating conditions, unreliable."
What are the surprising causes of Tube delays at stations?
Mr Jones said: "Surprisingly, it is the little things that can have the biggest consequences."
He added: "Ball point pen caps, coins, newspapers and bottle tops falling into the door slots and preventing them from closing are a regular cause of trains being unable to move.
"During the winter, scarves, coats and bags get caught in carriage doors and also customers leaning on them or holding the doors open can prevent a train from moving or suddenly jolting to a halt shortly after departing a platform.
Which line has the trains with the highest top speed?
Trains on the Metropolitan line reach the "absolute" top train speed of around 62mph on the outer sections, according to Mr Jones.
Central line trains come "very close" to this on the outer sections of the line.
Which trains can accelerate the fastest?
Mr Jones said: "Victoria line trains are the quickest to accelerate, with four AC modern motors driving each eight-carriage train."
Is there one button that turns off power to the whole network?
Mr Jones said: "There isn't just one button to push or switch to flick. Every night just after 1am, the 'juice' is turned off to most lines to allow engineering work to be carried out. It is turned on again at around 5am."
