Cardiff City fans travelling to Wembley by train are being warned they will risk missing the game if they misbehave.
On the eve of the Championship play-off final, transport police said anti-social or intimidating behaviour will not be tolerated.
Extra officers will be on duty on trains and at stations, and alcohol has been banned from services.
Up to 40,000 Cardiff fans are expected at the game against Blackpool.
Those travelling by road have already been warned of potential delays with plans to close a section of the M4 in Wiltshire on Saturday night forcing drivers to take a detour on the way home.
British Transport Police's sector commander for Wales, Ch Insp Sandra England, said: "Saturday promises to be a great day of football for Cardiff fans and our officers will be helping everyone to enjoy it, as well as ensuring fans travel to and from the game safely
"Throughout the entire day, we'll have officers patrolling stations and on trains between south Wales and London and they will not tolerate intimidating or anti-social behaviour from those intent on spoiling the day for others.
"Trains are confined spaces and it can be a very intimidating experience for families and children when they find themselves in a carriage packed with boisterous fans.
"Of course, we want fans to have a good time and enjoy the big day and we appreciate the vast majority are always well behaved but we are warning anyone who steps out of line while travelling on the rail network will be dealt with by our officers.
"Not only are they risking a court appearance, they also risk missing out on seeing their club's biggest match for decades.
"Alcohol will be banned on both the forward and return train journeys and is banned on the Tube at all times."
First Great Western's head of security, Paula Durrans, said: "We are expecting high levels of fans to be travelling to the match with us and we want everyone to have a safe and pleasant journey.
"Although there will be extra trains to get fans to the match, we would advise people to book their seats in advance for the most comfortable journey possible."