I respectfully disagree :) In real-life you can exceed the line speed limit at the risk of derailing or damaging equipment. You can also exceed the train's designated maximum speed at the risk of derailing or damaging the train. You shouldn't do either though. In the case of the 1st gen DMUs the max safe speed is probably limited by the power train, but for safety reasons, many other trains have lower max speeds than what they could actually reach.