New train operator South Central will keep ticket barriers at Sussex's main stations operational all day in an effort to crack down on fare dodging.
The strict new regime means it will be impossible to get on or off a train at stations with the automatic barriers without a valid ticket.
Barriers at Brighton, Hove, Lewes and Eastbourne stations have been left open late at night in the past, when there were not enough staff available.
South Central, which is owned by Govia, started to staff the ticket barriers at Brighton station from the first to the last train, a week after it took on the franchise on August 26.
In the first week, £3,000 was taken between 11pm and 4am at the special pay point set up for travellers who arrived at the station without a ticket.
Hove is now fully staffed and similar barriers at Lewes and Eastbourne will be fully staffed in the coming weeks.
A spokesman for South Central said: "The vast majority of passengers travel with valid tickets.
"Now it will be impossible to get through the barriers without a ticket so everyone should be warned that to travel on our services you must purchase a ticket and you will not get away without paying."
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