Engineer's instructions

Under a FIDIC Contract, any change to add or omit work must be made by an instruction from the Engineer under the appropriate sub-clause. You mention Sub-Clauses 51 and 52, which are from the 1987 4th Edition of the Contract for Civil Engineering Works. Sub-Clause 51.1, item (b) allows the Engineer to issue instructions to omit work, but this is qualified by the statement in brackets "(but not if the omitted work is to be carried out by the Employer or by another contractor)". Your proposal for the crane rails would appear to contravene this requirement. Hence the change and price adjustment may need to be negotiated between the Parties and would probably involve legal advice. - See more at: