Neither is a problem, leftover cheese will just get eaten as a snack if it’s not enough to keep until I go to the shop next (I usually go every other day for something since it’s a 2 min walk)

Leftover bread just becomes a piece of toast with some butter (I basically always have it in, and get the replacement before I totally run out). Tbh with a coffee that’s just my normal breakfast some days anyway

You always have a toaster and butter on the train?
No need, if you have a normal kitchen at the destination.