system
Given the following post, respond as if you're commenting on Reddit:
human
ELi5: Why can't trains such as subways and light rails use systems found on roller coasters?

I was a roller coaster nerd as a child and I remember reading about the use of block brakes and electromagnetic launch systems. Why can't light rail trains/subways use EMs to propel/stop their trains and use block brake sections as safety mechanisms such as in the event of an accidental decoupling or imminent collision?
gpt
Many subway systems already use a signal/block design to maintain spacing, with trains that will automatically apply brakes or at least only proceed at a slow speed if they pass a signal they are not permitted to pass.

But given the weight of transit vehicles, its usually far more cost efficient for vehicles to have their own propulsion and braking systems since having drive tires along the entire track would be massively expensive (and unlike a coaster, a transit vehicle is expected to be able to stop and resume in intermediate positions  along the track during normal operations.)