6.—(1) subject to paragraphs (2) and (3), a person shall not operate, and an infrastructure controller shall not permit the operation of, any Mark I rolling stock on a railway.
(2) Paragraph (1) shall not apply to rolling stock, which at the relevant time is being exclusively operated other than for the carriage of fare paying passengers.
(3) Paragraph (1) shall not apply to Mark I rolling stock which has been modified so as to ensure that in the event of a collision—
(a)The underframe of one vehicle will not ride over the underframe of another vehicle so modified (whether or not the vehicle is part of the same train);
(b)Where sub-paragraph (a) is not practicable, the extent of any such riding over is as small as can practicably be achieved by a modification to the rolling stock.
(4) In this regulation “modified” means modified by the installation of interlocking devices on vehicles and “modification” shall be construed accordingly.