We did a couple of journeys like this with DC1 as a baby, but older than 5/6 weeks (probably started travelling closer to 3-4 months old, at 5 weeks we were still getting our heads around the night waking, regular feeding etc so didn’t really want to entertain going away!).
We used a really big rucksack - like a backpacking type of rucksack - to fit in my own clothes and baby’s clothes, some toiletries, a couple of soft toys, that sort of thing. I only packed some nappies (eg 10) as it made more sense to pick up a pack at my destination rather than lug packs on the train. I did the same for formula - but if you’re breastfeeding then you don’t have to think about that (hurrah!) - also if you are formula feeding then I took Milton tablets and did cold water sterilising, I would ask whoever I was visiting for a tub or big bowl and do it that way. This kept down the amount of luggage.
I had a smaller bag at the top of the rucksack which I took out for the journey, eg a couple of bottles, nappies, wipes, travel changing mat etc. I carried baby in a sling/carrier.
Trickiest part was actually nappy changing on the train as not all trains are set up for it in the toilets, so it meant changing in the vestibule (ideally not soon before stopping somewhere!) then using the toilet for washing hands and disposing of nappy in the bin. Again all significantly easier with a sling/carrier.
I had a compact buggy which was folded and by luggage for the journey. I kept baby on me either in the sling or on my lap (sling in particular for napping). I can’t think of any cross country trains that are set up for having a pram unfolded for the journey, there just isn’t the space.
Some people will look at you with sheer fear to see a baby joining a long train journey, don’t take it personally
DC1 was quite a reliable sleeper and not loud, I was actually congratulated by two other passengers at the end of one 4.5 hour journey as DC1 hadn’t been disruptive at all and they were convinced it was going to be an awful journey!