Military duffles. If you don't have any ex-military in the family, go to a surplus store. They're recognizable, can be packed with as much or little as required, are sturdy as all get out and almost impossible to overpack as long as it's only clothes, shoes and bedding (mine was 49.5 pounds once, every other time it's varied between 42 and 47). They last forever, fit all train, bus and plane size restrictions. Best of all, when they get dirty, turn them wrong-side out and clip the hook to the tongue, and they're washable in any laundromat.