My backpack has a capacity of 70 liters. A standard backpack typically holds about 30 liters. It features wheels and a pulling handle. Usually, I pull it on wheels, but if the path is uneven or covered with dog waste, I lift the backpack onto my back.
A backpack this large shouldn’t be packed too full. It’s hard to find stuff in there. It can also be too big for sleeping cars on trains and other cramped spaces.
Nowadays, some trains have restrictions on the size of luggage/backpacks. For example, the luggage size for the Spanish Iryo train company is linked to the tickets: Baggage Policy | iryo