Also you can do the same thing horizontally, with more passengers and over greater distances. GoA 3 services from the 1980s and then GoA 4 services a bit later
[Grade of Automation 3 services don't need a human "driver" but they're not safe enough that we can just leave them to be used unattended like an elevator, there's a trained member of staff somewhere, probably helping tourists or checking tickets or something - the GoA 4 services are unattended, like an elevator there are humans but they're far away in a control center and you're safe enough while you wait for a human to come to where you are and fix whatever extraordinary problem the automation couldn't handle e.g. there's no electricity so you can't go anywhere.]
[Grade of Automation 3 services don't need a human "driver" but they're not safe enough that we can just leave them to be used unattended like an elevator, there's a trained member of staff somewhere, probably helping tourists or checking tickets or something - the GoA 4 services are unattended, like an elevator there are humans but they're far away in a control center and you're safe enough while you wait for a human to come to where you are and fix whatever extraordinary problem the automation couldn't handle e.g. there's no electricity so you can't go anywhere.]