But the project's current phase also includes replacement of five two-track bridges, which required reducing the number of active tracks from four to two.

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The Air Force is also in the AoA phase in its search for a replacement for the T-38, which was the world's first supersonic trainer when it entered service in 1961.

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The five-segment booster is intended to power the early flight phase of Nasa's proposed Ares 1 launch vehicle - its replacement for the space shuttle.

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