Before revision: At underground or indoor platforms, prevent the air from being polluted by the fumes caused by friction from the train wheels grinding against the tracks.PDF on the issue (London Underground): Personal Exposure and Air Quality in Northbank Their primary disadvantage is their cost ; installing a system typically costs several million USD per station .
After revision: At underground or indoor platforms, prevent the air from being polluted by the fumes caused by friction from the train wheels grinding against the tracks.PDF on the issue (London Underground): Personal Exposure and Air Quality in Northbank Their primary disadvantage is their cost .