Check at your library about ebooks. I used to download all sorts for a trip as all free and no excuses of 'I've read it, don't like it, bored of that story' as always a mix on there.
The I-Spy on a Train Journey book. We had so many of them for every possible experience that could involve boredom :) Or, can make your own. Write a list of things you think you might see (or they think they might see) and then have to keep an eye out for them on the way.
We still have a few of those pen and paper game notepads lying around - a pad of games like hangman, shut the box, word searches, etc.
For the tablet - Toca Boca games are good ones they can do together at the same time and should amuse both age groups. Look for the Town/City/Village ones - something like that rather than the haircut or doctor one - though, they are all good! There are a couple where there isn't a 'game' per se, but the kids can just wander around a colourful world doing things like making food, going in a lift, putting things down the loo.... :) It's built for more than one set of paws on at a time and there are no rules so it's great for them to explore and just see what they can do - so should be fairly calming and can be picked up by one or the other and not really messed up as there is a reset button (useful for when one of them has discovered they can remove all the movable objects and hide them in the storage room in another part of the game 
A magazine bought at the station - one with free plastic (terrible, I know!) things that means they can play with the toy/make whatever it is and read the magazine.
There are also kids puzzle books, which are good as have a mix of stuff.
Overall I would say a variety is the best way. Have had some journeys where DC have sat reading a book the whole way apart from lunch and a trip to the loo. But, other times have ants in their pants and want to do something different every ten mins.