Where the undertaking is intended to be carried into effect by a capital to be subscribed by means of the promoters of the undertaking, the whole of the capital of the company or estimated sum for defraying the expences of the undertaking shall be subscribed under contract binding the parties thereto, their heirs, executors, and administrators, for the payment of the several sums by them respectively subscribed, before it shall be lawful to put in force any of the powers of this or the special Act, or any Act incorporated therewith, in relation to the compulsory taking of land for the purposes of the undertaking.