The Central Line is a line of the London Underground and coloured red on the
tube map. It is the second busiest line on the Underground after the
Northern with 183,512,000 passengers per annum. It is a deep-level "tube"
line, running east-west across London, and has the greatest total length of
track on the Underground (but is not the longest continuous line)

Just read the above on Wikipedia....what is the longest continuous line it
is referring to?

Thanks.