Backpacks are typically less elegant to store in a luggage compartment because of their irregular shape and loose straps. The Eurolines luggage policy reads:
Unless indicated otherwise in the timetable, the general luggage allowance is two average sized suitcases (70x80x20 cm with a maximum total weight of 30 kilos) and one single hand luggage per person. (http://www.eurolines.be/en/luggage)
They don't seem to prohibit backpacks, but they don't explicitly allow them neither.
Does anyone have experience with traveling by backpack on a Eurolines bus?