Section 23. The court having jurisdiction of a trust created by a written instrument may, upon petition and after notice to all persons interested, if upon hearing it appears to be for the benefit of the trust estate, authorize trustees to mortgage any real estate held by them in trust for the purpose of paying assessments upon the trust estate for betterments or for the expense of repairs and improvements on such estate made necessary by such betterments or by the lawful taking of such estate or of a part thereof by public authority; for the purpose of paying the expense of erecting, altering, completing, repairing or improving a building on such estate; or for the purpose of paying the expense of other improvements of a permanent nature made or to be made on such estate; or for the purpose of paying an existing lien or mortgage on such estate or on a part thereof; or it may authorize such trustees to make an agreement for the extension or renewal of such existing mortgage.