The genuine hiking backpack is tall and narrow, making them ideal for train travel. The major disadvantage over travel packs is that you sometimes have to unload everything to get at your stuff at the bottom of the pack. Also the inability to lock your pack up raises some security concerns, although all backpacks are usually easy to slash with a razor, so that a padlock is hardly very good security anyway. With a small backpack it's not such a problem getting stuff from the bottom of your pack, especially if you get in the habit of rotating your dirty clothes to the bottom of the pack, so you always have what you need at the top. Stuff you use frequently can be kept in handy side pockets or a daypack.
Many travellers also take a day pack .
Although many people manage to travel for months, living out of a daypack, a day pack is mainly used for carrying stuff around town while your backpack is left in a hostel or the baggage locker at the station. A day pack needs to be big enough to hold important stuff like a guidebook, camera, a raincoat or sweater and a few snacks. Some travel packs come with zip-off day packs, however this is a big security problem since most travellers keep their most valuable stuff in a daypack and then zip it on the back of their backpack where it is out of sight and easily opened by pickpockets. Make sure that your backpack is big enough to hold a fully packed day pack.