The line was opened in 1866 to carry slate from the quarries at Bryn Eglwys to Tywyn . It was the first narrow gauge railway in Britain authorised by Act of Parliament to carry passengers using steam haulage . The line was opened in 1866 to carry slate from the quarries at Bryn Eglwys to Tywyn , and was the first narrow gauge railway in Britain authorised by Act of Parliament to carry passengers using steam haulage .