Frames

Locomotive frames from that era were almost always made of cast steel. Works about the same way at the foundry, except that they pour molten steel into the mold instead of iron, and it is considerably more expensive. In most cases, if you can find one that is clean enough, you can see the casting numbers in the non machined portion of the frame, say in the opening for the equalizer. The metal is easy to weld. The problem comes from trying to keep the frame from pulling out of allignment as the weld cools. You can turn your engine into a pretzel quite easily if you are not careful.
A frame's lifespan is rather subjecive. As the engine ages, the continual stresses lower the fatigue strength of the frame, and it breaks more, and more often. The frame's life is over when the railroad decides that it is sick and tired of having to take wheels out and so on in order to rerpair the cracks that continually show up.
Now days, it is a no-brainer to use flame cut plate to replace a worn out frame. The hard part is taking the engine COMPLETELY apart, and then, putting it back together again.