A high-speed rail link between Scotland and the south of England could be built in just five years if it was made a top priority, MSPs were told.
Colin Elliff, a chartered civil engineer, claimed the construction work could take five years. But he said the timescale would depend on the priority the project was given.
Mr Elliff argued: "If we are faced by a world where CO2 emissions have suddenly become critical and where oil supplies become prohibitively expensive, the priority could vastly escalate." He added: "In sheer physical terms it could be built in five years."