How to entertain kids on long journey

[Ahhffs]

We have a very long train journey coming up and I'm very nervous!
They're not little kids (12 and 9) but they are very very energetic kids with a pretty low boredom threshold.
12 year old has adhd and asd and has never sat and read a book in his life, im not convinced he's even ever actually sat still for any real length of time for that matter! 9 year old will happily read a book or whatever for a good while but probably not 5 hours
any ideas that have worked for anyone else?

Teach them a card game and play it with them. Drawing and writing, games of consequences.

Chat about where you’re going and other plans for summer hols
Travel/magnetic games like mini chess, mini Connect 4 (you can buy them on Vinted quite cheap usually)
Uno
Headphones and a choice of movies downloaded to watch

Does the 12 year old have games on his phone or a handheld gaming device?

Games like Dobble are always good, Lego kits or other non-messy art stuff, travel board games, magazine of their interests, puzzle books (Sudoku, logic puzzles, etc), just a plain pack of cards.

My grandchildren were younger than yours when we did a big trip but I googled travelling games and printed off a load of stuff, bought a few recommendations, packed snacks and something for earphones and we did pretty well on a twenty four hour door to door travel.
You might want to get familiar with coping strategies for the terminally restless as well and start those straight away.
Boredom is a lifelong condition and the sooner we accept responsibility for it the better. As a parent you are not a 24/7 performing clown.

A pack of playing cards, colouring books or wordsearch? Mine always used to read in long train journeys