It will be paid in two instalments linked to progress on construction of the Stockley Works, which will deliver greater capacity and reliability for rail services into Heathrow.
In return, Crossrail will guarantee a fast train service four times an hour for the majority of the day.
Mayor of London Boris Johnson said: ‘This is another significant step forward in preparing for the construction of this landmark new railway. All parties are committed to Crossrail's success, and together we will deliver it on time and on budget.’
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