S.8  DISPERSALS
In land areas where there would be dispersals at the end of the turn, if there are any supply trains present then the number of dispersals is reduced by the number of supply trains and one supply train is used up.

S.9  SUPPLY COSTS
The supply cost for each area is the same as the maintenance cost for the area, except that supply costs are reduced if there are supply trains present. Supply costs are calculated and deducted along with maintenance costs (at the same time).
In land areas where there would be a supply cost of more than 1 BP at the end of the turn, if there's more than one supply train present then the supply cost is reduced by the number of supply trains and one supply train is used up.

S.10  SUPPLY CAPACITY
Each empire has a supply capacity which limits the size of supply actions. It is fixed at the start of the game, and cannot be changed. Empires not in the list below have a supply capacity of 2. 
England  12     France  8     Austria, Russia  5     Prussia  4     Poland, Spain, Sweden  3

S.11  BUILDING SUPPLY TRAINS
The GATHER action builds supply trains in a supply centre. The number of supply trains that can be built in a single action is limited by your supply capacity and also by the population in the area. You can't gather supplies in a base that is blockaded (ie. one where you don't own the sea area outside), Supply trains cost 1 BP each to build. The "ALL" and "ALL BUT" formats are allowed.
[ GATHER ]  [ WHERE]  [ blank ]  [ NUMBER ]   to build new supply trains

S.12  LEFTOVER BPS
If the number of supply trains in your capital is less than your supply capacity then any BPs left over at the end of turn are now spent first on building supply trains in your capital (as many as is allowed, according to your supply capacity).