Lets say you have a K-4 with a cast boiler and another with a "wrapper" style. What is the difference in weight?

Jim--

I'm not sure I understand the point of your question. Locos with sheet brass boilers naturally weigh less than locos made with cast brass/bronze, or zinc alloy boilers. The difference might be as much as a couple of pounds. But almost any loco can have weight added to some extent, depending on the design of the chassis & drive mechanism & strength of chassis & drive components.

Sheet brass models often come with boiler weights (either separately as supplied by Max Gray & US Hobbies, for example, or factory-installed as supplied by other mfrs). Weight can be provided with materials of different density as desired to fit space & budgetary considerations.

Personally, I don't think weight is as much of a consideration in buying model locos as other points such as the amount of, or accuracy of details included in the model, operating performance, quality of finish & workmanship, etc.

If everything else is satisfactory (including added weight), and the loco still isn't heavy enough to pull a desired train, my suggestion is to either lighten the load (reduce the number of cars) or doublehead!

I realize they add weight. I want to know how much? The models I get hold of are fine for 8-10 pieces of rolling stock, but start talking about 15 passenger cars and a lot of models get weak.
I am considering trying my hand at building a kit and want a baseline to work from.
Also like to hear more about WHERE they add weight. On the frame, in the boiler, cylinders.

For a rigid frame, two cylinder engine, you can add weight above the driving wheels. For an articulated engine you can add weight above the rear drivers only because the front drivers are usually not firmly attached to the boiler (Scott Mann told me this). Make sure that your motor can handle the added weight. Anytime you add weight, you put more strain on your motor, gear boxes and driving mechanism. You should only add weight only if you really think you need it. I suggest you widen your curves, get a bigger engine (MTHs large engines are excellent pullers). My MTH N&W Y6b pulled over 100 loaded coal hoppers on an O-72 curve. The AC-6 cab forward has been tested to pull in excess of 220 cars (see a recent issue of Classic Toy Trains or O-guage Railroader for a review of this engine). When you add weight to a brass engine, this makes handling them more difficult and small parts can be knocked off.

Jim----It is an old rule of thumb that you can add weight to your locomotive but not so much that the wheels can't slip. There are other considerations though. I used to haul a 15 to 20 car train with a Weaver brass 0-6-0. Some of the cars were Lionels and rather hard rolling. Well it gave out. It seems that the pillow block suporting the drive shaft tore loose from the mainframe. The locomotive was returned to Weaver for repairs but when I got it back I decided not to break its little heart by hauling such a heavy train so I stopped hauling such heavy trains. Odd-d

I personally like long trains. A Pacific pulling 8-10 cars works but a Northern should pull 12-16 cars. If weight is needed, add it.