In most subways,there are stations
about every one kilometer along the route.
Most subway are very modern and clean,
but they can get very crowded sometimes.
So,people have to follow certain rules
to make the trip more comfortable.
In London,when people use the Tube,
they must stand on the right
so that people in a hurry can walk on the left.
They let passengers get off the train
before others get on,
and people are expected to give up their seats
to old and disabled people.
Some subways provide frequent services.
Trains arrive at a station
about every two minutes.
Subways carry a lot of passengers.
The number of people traveling oneway each hour
in London can be about 30,000.
However,in some cities it is much higher.
Hong Kong's subway holds the record
of about 80,000 passengers an hour.