superlative form of soon: most soon
"The earliest train arrives at the station soonest, making it your best option to catch the deadline."
In plain English: Soonest means happening at an earlier time than anything else that could possibly happen.
"As soon as you arrive, we need to get our earliest tickets for the flight that leaves at noon tomorrow morning."
superlative form of soon: most soon
"The train that leaves at six will get us to the station sooner than any other option, making it our soonest arrival time."
In plain English: Soonest means happening at any time earlier than all other possible times.
"We will leave for the airport at the soonest time possible to catch our flight."