RCM on Royalty Charges - Goods and Services Tax - GST

We party is a civil engineering contractors, executing works for railways. They are using soil for embankment, though they have not taken any lease from mining authorities, however as per rules royalty is payable on extraction of soil. They are not paying royalty directly, but the railways is recovering the amount of royalty due from their bills and depositing this to revenue authorities. The soil extracted is being used in works which are chargeable to the GST. Is the contractor liable to pay GST under RCM on royalties paid by the railways on account of the work, executed by them, which is charged to GST on the full value, including the royalty amount. Please guide, Is the