When waiting for the underground, I've always been wondering why we could not absorb the kinetic energy of the train to slow it down at the station and then to release this energy to help it accelerating during the starting up. This would also reduce air pollution due to braking at underground stations.
Indeed, in frequently-stopping, trains such as underground trains, most of the energy is used to accelerate, so I am wondering whether the train could be slowed down by a large spring that would be compressed while arriving a stop station, then blocked when the train is stopped for the travelers to get out/in, then released to help the train accelerate to continue its route.
Of course, the train should not go to the opposite direction when releasing the spring, so a system should be designed so that the spring can be released in the correct direction.
This idea looks simple and I'm sure engineers has already thought about it, however, I've not found any discussion on this on the web. I found that on some Spanish lines, the kinetic energy during braking is converted to electricity and used by other trains from the same line (research paper), but why not keeping the mechanical energy to release it directly with a spring method?