The tunnels were due to the elimination of the piston effect, which is unpleasant to passengers in the stations, built as double-track.
ðə tˈʌnəlz wɜː dˈuː tə ðɪ ᵻlˌɪmᵻnˈeɪʃən ʌvðə pˈɪstən ɪfˈɛkt, wˌɪtʃ ɪz ʌnplˈɛzənt tə pˈæsɪndʒɚz ɪnðə stˈeɪʃənz, bˈɪlt æz dˈʌbəltɹˈæk.