Okay, so maybe I watched Titanic one too many times, but in my Traveller universe there is a passenger class between middle passage and low berth - steerage. A typical steerage compartment consists of a series of triple bunks, a communal galley/lounge, shared fresher facilities, and additional life support equipment. Quality and accommodations may vary widely from ship to ship, but the Imperium does have, and sometimes even enforces, minimum standards for passenger well being.
At minimum, each passenger is assigned a single bunk with a small storage locker - usually enough space for a suitcase or two. Fresh linens are provided at the start of the trip and changed weekly. Two meals per day are included, but are often not much more than unheated survival rations with appetite suppressants. Passengers are encouraged to bring their own food if they have special dietary needs. Access to toilet facilities is unlimited, but shower facilities are usually limited to once or twice per week. Bunks often only have a simple curtain to provide privacy, but some may provide a more secure, hardened and lockable screen panel.
Steerage passengers are often required to stay within the confines of the steerage compartment, except in the case of emergencies. Depending on the number of steerage passengers, the passenger's ticket price, and ship's amenities, some passengers may be allowed access to other areas of the ship during off hours. Steward services are not included, but a good captain will have a steward - or security personnel - do a walkthrough of the steerage compartment once a day to check on things.
Larger passenger liners may have a good portion of their ship dedicated to steerage compartments. With the increased passenger load per square meter, a shrewd captain can actually make more money with steerage passengers than with high passengers, although usually with considerable more headaches. The additional wear and tear of having large groups of people in a small area means stateroom fittings need to be replaced more often or more durable, and therefore more costly, fittings be installed in the first place.
Steerage compartments are generally open to any passenger, but some may be segregated by sex, alien species, families, etc. It is not uncommon for some groups to book an entire steerage compartment for their private use; corporations moving employees and their families to a new facility on another world, religious factions on a pilgrimage, student groups, traveling entertainers, or soldiers. Travelling steerage is a great way to meet a wide variety of travellers in a short period of time and a great source for news, rumors, and potential jobs.
Steerage passage typically costs between 2,000 and 4,000 credits per trip and is usually a "you get what you pay for" proposition.