If any permanent or temporary alterations or additions to railway property are reasonably necessary in consequence of the construction of a specified work, or during a period of 12 months after the completion of that work, in order to ensure the safety of railway property or the continued safe operation of the railway of Network Rail, such alterations and additions may be carried out by Network Rail; and if Network Rail gives to the undertaker reasonable notice of its intention to carry out such alterations or additions (which must be specified in the notice), the undertaker must pay to Network Rail the reasonable cost of those alterations or additions including, in respect of any such alterations and additions as are to be permanent, a capitalised sum representing the increase of the costs which may be expected to be reasonably incurred by Network Rail in maintaining, working and, when necessary, renewing any such alterations or additions.

If during the construction of a specified work by the undertaker, Network Rail gives notice to the undertaker that Network Rail desires itself to construct that part of the specified work which in the opinion of the engineer is endangering the stability of railway property or the safe operation of traffic on the railways of Network Rail, then, if the undertaker decides that part of the specified work is to be constructed, Network Rail must assume construction of that part of the specified work, and the undertaker must, notwithstanding any such approval of a specified work under paragraph 22(1), pay to Network Rail all reasonable expenses to which Network Rail may be put and compensation for any loss which it may suffer by reason of the execution by Network Rail of that specified work.

The engineer must, in respect of the capitalised sums referred to in this paragraph and paragraph 27(a) provide such details of the formula by which those sums have been calculated as the undertaker may reasonably require.

If the cost of maintaining, working or renewing railway property is reduced in consequence of any such alterations or additions, a capitalised sum representing the saving must be set off against any sum payable by the undertaker to Network Rail under this paragraph.