There are now in the Tehran metro only 217 trains, with 1514 wagons.
For the Tehran metro, 791 new trains are a lot. But 791 new wagons are ok.
What you refer to as train is locomotive or the powered wagon. It is an old style metro equipment. China does not operate or produce such equipment anymore. So the 791 from China are units, some are powered, some are not. The concept of powerless "wagon" can not be used on this deal. It is not 791 powered train-head, neither 791 powerless carriages.
This is what I have been saying in my post 178, it is wrong to apply the terminology of "locomotive and wagon" on this 791 deal. It is also wrong to use the word train to refer to the train-head.
Also I must point out that the wiki page is misleading in saying "217 trains and 1514 wagons" instead of total units. If you look at any metro line you will see that all carriages are the same except one in the head and another in the tail that have driver's cabin. The motors are all under the floors and can be located anywhere along the whole train set. Metros have to travel up and down due the tunnel system, so it has to place some powered carriage in the middle. Which carriage is powered is not visible to passengers, only the operator knows. In a deal of 791 it is impossible that all of them are powerless which would demand that Chinese wagons being connected in a train set with powered unit from other vendors which may not be technical possible or create extra cost of modification instead of using Chinese powered units.