Town growth works very different in Wasteland than it does in the standard game (and likely different than any other set). In normal OpenTTD, all that is required is that you shuffle passengers (or some other cargo) to and from towns and they will grow automatically. In this NewGRF however, it's not so easy, towns must be grown in stages by delivering various cargoes.
There are four stages of buildings, which appear in order: destroyed buildings, makeshift shacks, renewed buildings, and self-sustained buildings. Each stage of buildings accepts the cargoes necessary for buildings of the next stage to develop.
NOTE: All required cargoes must be delivered within the same 2 month period to be effective. The amount of cargo delivered does not matter, however.
On map creation only destroyed buildings are present, the empty rubble of the former towns is all that remains. Within these buildings reside a few wastelanders, but otherwise towns are virtually useless at this stage. Food and Water must be delivered to these remains in order to allow makeshift shack buildings to be built. This is the first step in rebuilding the town, as people begin to settle and construct rudimentary housing from the rubble. Once a few shack buildings are present, it will become possible to deliver a third cargo: building supplies.
These building supplies will make it possible for the residents there to build more sturdy, permanent structures, remiscent of the pre-war buildings. Once these so-called renewed buildings begin appearing, Passengers will also begin to appear in greater numbers, coinciding with a rise in the town's population. At this point Passenger transportation starts to become viable. After a sizeable amount of renewed buildings have been built, a new building type will automatically begin appearing, called self-sustained buildings. These buildings are similar to renewed buildings except that they no longer require any town growth cargoes, and only accept Passengers.
At this point, towns should begin to 'work on their own', no longer requiring you to deliver any special cargoes in order to grow**. Normal OpenTTD town growth rules do still apply however, so continued service in the town will be necessary to keep towns growing, but the hard part is over. Now you can start to set up your passenger network and kick back to watch things grow as usual! Congratulations, you have successfully rebuilt and saved a town in the wasteland! Just repeat until all the towns on the map are saved.
** For faster transitions it is actually best to discontinue town growth cargo delivery ASAP, and focus on passengers instead. If for some reason you wish to keep delivering town growth cargoes at this point you can, but over time towns will naturally grow out of accepting them. Once enough renewed and self-sustained buildings exist in a town, shack buildings will inevitably be pushed out by the more up-scale population. Once that happens, building supplies can no longer be delivered, leaving self-sustained buildings as the only ones that can be built. This transformation can only be delayed by continuing delivery of other cargoes.
Quick Tip: If you can't tell which buildings are which, enable transparency (by pressing x). The house tiles are color-coded to make indentifying each building type quick and easy.
Red - destroyed
Yellow - makeshift shack
Green - renewed
Blue - self-sustained
Also of note: new houses check for cargo acceptance within a certain distance, NOT just within their own town, so it sometimes happens that buildings will appear in an adjacent town to the one you're trying to grow. Conversely, having a station accepting cargo in a town may not affect the entire town, only the part closest to the station (The area houses check is quite large though, so with the exception of very large towns created in the scenario editor, the latter case should rarely happen [with decent station placement]).
The one exception to this behavior is self-sustained buildings, which rely on counting the amount of renewed buildings to decide when to appear. This count IS done on a per-town basis, so renewed buildings nearby in an adjacent town will not be detected; they must all be in the same town. Keep this in mind if you plan on trying to grow two close towns with the same station.
Town Growth: Cargoes
Hint: It is best to wait until you have all necessary town growth cargoes available and begin delivering them at the same time, rather than only delivering one as soon as it's available, otherwise you may actually cause the town to shrink!
The Food cargo is actually split into two types: Irradiated Food and Clean Food. Irradiated Food is the
easier-to-obtain but less desirable food type, and is only accepted by destroyed buildings. Since shacks and renewed buildings will NOT accept this cargo, it can only be used to grow towns to the second stage, and
isn't of any use in the later stages of town development. The Irradiated Food chain is much easier and cheaper to set up however, so it can be useful early in the game or for temporary use.
Clean Food on the other hand is much safer and healthier, and all building types accept this cargo (except self-sustained buildings). Clean Food is necessary to grow towns to the final stages, so it is much more beneficial in the long run to deliver this instead of Irradiated Food. However, there are several additional steps necessary to get production going, so it is more expensive and complex to distribute and may take some time to set up.
Water is an essential part of life, and in the wasteland it is tough to come by clean sources of it. Only at special deep-well pumps can fresh water be extracted and delivered to towns to allow them to grow. All town buildings accept water, and it is a requirement for all stages of town development.
Building Materials are necessary for renewed buildings to develop, and as such are only accepted by makeshift shacks. Once building materials are successfully delivered to a town (in addition to food and water, of course), it allows the construction of renewed buildings to take place. Once a town has built enough renewed buildings, self-sustained buildings will begin appearing and ALL town growth cargoes will eventually no longer need to be delivered.
Unlike wastelanders, which are simple people just trying to survive, passengers are more refined and are used to the comforts of modern society. Only residing in renewed buildings and self-sustained buildings, passengers are the toughest cargo to get in the game, since towns need to be rebuilt first in order for them to appear in great enough numbers to be useful. Once available, passengers act just like they do in standard OpenTTD; they can be transported from city to city and keep towns growing after the other cargoes are no longer required. Passengers are only accepted by renewed and self-sustained town buildings.
Although not technically a town growth cargo, wastelanders can play a part in how towns grow. They are found in small numbers in destroyed buildings, but will start to become more plentiful once shack buildings start to appear. Their primary purpose is to be transported to certain industries to provide a production boost. The increased output from these industries provides an often much-needed boost of income and supplies to your network, so it can be tempting to start transporting wastelanders from a nearby town right away. However, be aware that transporting wastelanders from a town without supplying any town growth cargoes in return will ultimately cause the town to slowly wither away. Keep this in mind when planning your strategy!