Many large cities have an underground railway, or metro, to help people move around without traffic jams. The first underground railway opened in London in 1863. Today, cities such as Cairo, Dubai and Riyadh also have metro systems. A metro train can carry thousands of passengers in a short time and is not stopped by traffic on the road. Since fewer people use cars, the air is cleaner and the streets are quieter. Building a metro is extremely expensive and can take many years, because workers must dig tunnels under buildings. Even so, many governments think the money is well spent. Passengers can read, rest or study on the journey instead of driving, and they avoid the cost of parking. For these reasons, cities that grow quickly often plan new metro lines.

Question: Why does the writer say that building a metro is extremely expensive?