Transport Secretary Patrick McLoughlin said the current weekend disruption on rail networks shows why upgrading the current rail system is not a viable alternative to the HS2.
He told ITV Daybreak the delays and disruption that occurred during the £9 billion upgrade to the West Coast Main Line proves why a new system is preferable to trying to increase capacity on the current network.
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Extracts released by 10 Downing Street suggest Mrs May will say the UK will not settle for a "half-in, half-out" policy.
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