Passengers often form the mainstay of a company's revenue - at least in the early days. Passengers can be transported from town to town or within towns using either trains, aircraft, buses or ships. The larger the town the more passengers it can provide for your transport systems.
Contrary to popular belief the transportation of passengers between towns is not necessary for the growth of a town, but as active stations improve town growth, transporting passengers within and between towns is one of the most effective ways to grow towns.
Passengers don't have their own will and behave as any other passive type of cargo. They don't create a directional flow. There is no notion of people desired destination or path in the game, nor there is any particular "pull" demand for more people. Passengers simply hop on any suitable vehicle and drop off wherever the vehicle is ordered to unload. Thus, standard play strategies apply when passengers transport system is designed. E.g. transfer them for an optimal distance, not destination; passengers don't pay for variety nor reach.
In the early stages of the game it is sensible to use buses to transport passengers between towns. Using buses keeps initial set up costs and running costs low and allows for a generally reliable revenue stream.
Buses are not always effective for transporting large amounts of passengers as they do not have a large capacity so many are needed to satisfy demand - raising costs and clogging up stations.
Once a town has grown sufficiently it is more economical to use trains to transport passengers. Trains are generally used for medium-long distance travel between large towns. Using a train allows you to carry a lot more passengers at once than on buses and keeps costs low. Trains can often travel faster than buses meaning that greater revenue can be gained from them. For a train to carry passengers a passenger car must be attached to the engine.
Aircraft are the most efficient way to transport passengers over long distances. All airplanes have a large capacity and travel at high speed - meaning high revenues and low costs. Supersonic aircraft (those with speeds of around 1400mph), though initially expensive, are often an excellent way to transport passengers as they have both a large capacity and travel at the highest quoted speed in the game - increasing revenues.
Aircraft can also be used effectively if it is difficult to build a road or rail track between towns (e.g. hills in the way) as the only infrastructure needed is an airport.
Helicopters are not often used to transport passengers (or anything for that matter) as they cost nearly as much as a plane and have very small capacities. However, they have been shown to be successful in transporting passengers between town centres. A heliport is built in a very central location in the town and helicopters are used between it to transport passengers and mail to other cities.
There are three main passenger ships in OpenTTD the MPS Passenger Ferry, FFP Passenger Ferry and the Bakewell 300 Hovercraft. Transporting passengers with ships between towns can only be done if the town has access to a canal or sea.
Ships have a low inital set up cost as they are cheap and require only a dock to be of use.
The ferries should be used for transporting passengers over large distances; they are slow but their large capacity makes up for it. However, if the ship is travelling long distances then buoys will need to be used.
The hovercraft is by the far best passenger ship - it holds a large capacity and is much faster than the two other ships. It can lead to large amounts of revenue very quickly. It should be noted that the "hovercraft" cannot travel over land - only sea.