Higher speeds reduce efficiency. The most efficient use of the tracks would be to limit all trains to the speed of the slowest train that travels a route. Then

Higher speeds reduce efficiency. The most efficient use of the tracks would be to limit all trains to the speed of the slowest train that travels a route. Then you can have minimal distances between trains. Also, faster trains are less fuel-efficient, quadratically with speed. So a slowdown would help the environment and the throughput. The only thing it wouldn't improve is passenger happiness ;)