In the discretion of the commission such bonds may be secured by a trust indenture by and between the commission and a corporate trustee which may be any trust company or bank having the powers of a trust company within or outside of the Commonwealth. Such trust indenture may pledge or assign tolls and revenue to be received but shall not convey or mortgage the turnpike or any part thereof. Either the resolution providing for the issuance of such bonds or such trust indenture may contain such provisions for protecting and enforcing the rights and remedies of the bondholders as may be reasonable and proper and not in violation of law, including covenants setting forth the duties of the commission in relation to the acquisition of properties and the construction, maintenance, operation and repair and insurance of the turnpike and the custody, safeguarding and application of all moneys. It shall be lawful for any bank or trust company incorporated under the laws of this Commonwealth to act as depository of the proceeds of the bonds or revenues and to furnish such indemnity bonds or to pledge such securities as may be required by the commission. Such indenture may set forth the rights and remedies of the bondholders and of the trustee and may restrict the individual right of action of bondholders as is customary in trust indentures securing bonds and debentures of corporations. In addition to the foregoing such trust indenture may contain such other provisions as the commission may deem reasonable and proper for the security of bondholders. All expenses incurred in carrying out such trust indenture may be treated as a part of the cost of maintenance, operation and repair of the turnpike.