Ahead of the game, Grand Central have issued the advice to anyone travelling with them to the capital.
They want to “ensure no fans are disappointed when trying to board one of its trains travelling between Sunderland and Kings Cross on the day”.
Sunderland AFC transfer rumours: Cats, Middlesbrough and Sheffield United 'interested' in Everton defender plus Tottenham face Jack Clarke decision
The strongest Sunderland XI Alex Neil could pick right now - and the clear transfer priorities it shows
15 eye-catching transfers made West Brom, Stoke, Middlesbrough, Norwich and Sunderland's Championship rivals
Contract details revealed as Sunderland star Anthony Patterson signs new deal in massive boost
Why Premier League clubs could be more reluctant to sanction loan signings as Sunderland monitor Everton and Tottenham situations
Sean English, chief operating officer for Grand Central, said: “We’re expecting a high number of football fans to choose to travel with us on Saturday 21 May, and we will be operating a ‘reservation only’ policy.
"This ensures customers who have a seat reserved on our trains, will get their seat, and we can successfully take them to London for the game and bring them home.
"We’re advising all customers to plan ahead and book before they travel and we wish Sunderland AFC all the best for the final!”
Sunderland have been handed an allocation of 37,480 for the play-off final against Wycombe Wanderers.
Tickets went on sale on Wednesday morning, available initially to season-card holders either for the current or next campaign.
Supporters will initially be limited to one ticket each, though they can be booked in a block of up to six.
All sales will be conducted online via ticketmaster, after advice from the EFL.
Prices begin at £30 for adults and rise up to £80.