Hi Hongbo, I'm so glad to read about the changes talking place in your city. It's really good news that you can travel by subway in Xi'an, though there's only one line there. Here in London we have twelve lines. By the way, we don't use the word "subway" here. It's American English. We call it "the Underground", instead. The London Underground is very famous. It's famous for several reasons. First of all, part of the underground railway today is the oldest in the world, which opened in 1863. Secondly, the London Underground is also the first line to have electric trains, in 1890. Thirdly but not the last, the London Underground is the second largest metro system in the world, after the Shanghai Metro. There are 270 stations in the Underground and it has 402 kilometres oftracks . What's more, the city of London itself helps make the transportation system famous! I've got a few photos to help you build a clear picture of the Underground in my city. They are together with this mail. In your email, you also said workers were building more lines. I'm sure your city is developing fast. Maybe you can send me a few photos of your city today. I really want to see the changes taking place there. I'm happy for you to live there in an ancient city with such a new look! So much for now. I'll write soon again. Best wishes! In the email, the words "subway", "underground", and "metro" mean  _  .
three different things two different things almost the same thing nothing like each other