The logic is the same he's slowing the country's economy down, because if people are hit by moving vehicles at 20mph they are less likely to die, so he could do the same with trains, as I said if you don't step onto the road you won't get hit.
Of course, there will still be the odd drink/drug driver, speed limits won't apply to lawbreakers, but they will still plow off the road and into people and kill them.