By locos you mean 1-3 wagons or 1-3 engines combined together 
, i dont intend to have more than one steaming mary engine in my western train, The Train engine, the coal wagon, the open wagon , 2 passenger wagons ,1 cattlew wagon and 1 cargo wagon plus the two cabooges , that is ( incl the coal wagon ) a total of 8 wagon, will the steam mary be able to pull this or it will go as a slug especially with the transformer i just bought ?
1-3 locos = 1-3 engines with motor or 1 engine with 2 motors + 1 with 1 motor.
The wagons/cars aren't relevant, cause they aren't electrical.
You need more power if you want to drive with many locos or with some very big locos like the US locos from Aristocraft, USA Trains and so on.
If you have only 1 or 2 or 3 locos, which drive at the same time, you don't need more power than 4375 has.
The power of the Steaming Mary is independent to the transformer (4375 or 4359)!!
If your track layout is flat, you could pull with it perhaps 6 (to 8?) wagons, but if there is a mountain (max. 2% to 3%), you can pull 3 wagons, not more...
Do you drive inside? There are not lots of mountains.
