Looking at the capacity of train networks, systems like CBTC help increase capacity, but the spacing of trains is ultimately limited by poor braking performance. In my understanding, safe distances are calculated by assuming a worst case situation ahead, such as a derailment or collision. The train behind must have adequate stopping distance plus a safety margin behind the train in front. Since trains have such poor braking performance, this required spacing is very large, especially for heavy and/or higher speed trains. Why aren't trains, especially in, for example, very dense subway systems, equipped with some auxiliary braking system, such as additional rubber shoes that could contact the track, or even maybe extreme things like plows off to the side of the track, that could stop the train very quickly in a rare emergency, such as the train ahead derailing?
Let me add a bit more detail about the thinking behind this question: Commuter trains in my area can brake at about 0.05g, so they stop in about 1/2 mile. Once you multiply out all of the various safety factors, they wind up requiring 10 minute spacing. By comparison, a bus driving down the highway maintains a following distance of less than 10s. Since the spacing for trains is based on a worst case scenario of an instantaneous derailment ahead, and braking distance is proportional to the square of deceleration, even a 40% increase in emergency braking deceleration is good for nearly a 2x increase in capacity on a track. 0.1g is unlikely to cause injury to passengers, and this is only applicable in the worst case catastrophic derailment ahead. Since this scenario almost never happens, even a single use brake that completely destroys the train and damages the track is acceptable as long as the passengers aren't injured.
Another example in my area is BART. BART's bottleneck is in the transbay tube, with one track in each direction. There's been talk about building a second tube, perhaps at a roughly 10 billion dollar cost. This would be necessary because BART says it can't maintain a headway of less than 2.5 minutes safely. If a bus can maintain a 10s headway safely, the only thing keeping the capacity of BART from being 15x better is braking performance in an emergency.