1.In big cities,the streets are usually very busy.So you can

1.In big cities,the streets are usually very busy.So you can f____ trains under the ground or on a rail above the streets.They carry people q____ in and out of the city.
Can a train run over water?Yes,it can.There are many b____ over the rivers.The trains can easoily run over the water .There also many tunnels u____ rivers.The trains can go through the tunnels swiftly.But it is not easy to b____ the bridgesor make the tunnels.Would you like to take a maglev train tin Pudong,Shanghai?It can speed un tp 431 km per hour.It is the ____ train in China.