In traditional train control, the railway is split into control "blocks" with signals in each one. Blocks are sized to allow the heaviest or fastest trains to stop fully within them. That way if a train is stopped in the next block, the following train will always have time to fully stop before reaching it. Careful tuning of block spacing is needed; if they are signals that are placed too close from each other, train speeds have to be reduced so they can still stop in time, but spacing them out further means the trains are also spread out and the route capacity drops.