The limitation for train capacity is the need to maintain a "block" to ensure that a train has enough time to brake in the case of an emergency, without hitting

The limitation for train capacity is the need to maintain a "block" to ensure that a train has enough time to brake in the case of an emergency, without hitting the train in front of it. For Japan's high speed rail, it takes ~2.5 miles to stop a train. On top of that, many railroads still operate with a static set of blocks, where the track is divided into areas called blocks that do not change over time. Only one train has essentially a lock on a block at a time. So a train has to own the block it is on, but also needs to get access to the next block early enough to avoid an emergency braking event. This reduces the capacity of a line. There's a movement towards "moving block" systems, where railroads use sensors to track train movement and create a moving buffer around the train, which should increase capacity.