I am going on a 2 day long train ride next week. The food on the train is a bit pricey for my taste. I will be travelling with a friend and several children. Do you have some ideas for food to bring... things that travel well?

Don't tell me noodle soup.

You can usually buy snacks through the window from local vendors when the train stops in stations on the way. Cheaper but always a bit of a risk, hygiene-wise, I guess.

"Several children" ! Maybe you could start eating those or exchange them for food with the local vendors through the windows.

A few years ago I travelled with a guy who arranged for Pizza Hut (I think) to deliver pizza and noodles to the carriage as we stopped at Nanchang train station. It went down a storm. However I remember he ordered me a ridiculous fizzy pop that had a scoop of ice cream in it. I can't imagine it tasting nice when it was freshly made let alone the state I got it in at the point of delivery.

sandwiches the first day
sardines + crackers
beef jerky
potato chips

On long haul trains, they have man pushed-carts with meals. They are usually 20kuai. Usually too salty and not gourmet food but keeps you alive.

Otherwise homemade sandwiches wrapped in a big salad leaf, will last for at least a day.

Usually KFC or some other fast food takeout for first meal is good

One meal can be nissin cheese instant pasta, add some fresh tomatoes. C4 or Walmart should carry it.

Fresh fruit is sold on the train for a not unreasonable price.

granola bars

Fresh fruit.
Yogurt, doesn't need refrigeration even if you eat it on day two.
Breakfast cereal, you can buy little boxes of UHT milk.
Canned fruit, meats, even Heinz beans are ok eaten cold.

peanut butter (with crackers or bread)
and tiger's baked beans is a good one.

break a leg.