I'm not sure if I missed something, but just how did they acquire a steam locomotive (which I would nickname "ZombPiercer")? It's not exactly straightforward to run one of these, I'm sure, and probably fairly easy to mismanage one enough to make it explode. But then, perhaps they're only doing a short run, and Chop probably has an encyclopedic knowledge of everything.
Still, I'm glad that they met up with the vaccine researcher/virologist and the one person in the world who has antibodies.
It's interesting to me that she has such antibodies, because normally it takes days or weeks for an antibody response to be built up. The zombie virus acts so quickly that those who are bitten become compromised before their immune system can do this. I would speculate that she must have encountered a fortuitously similar antigen in the course of her life that primed her immune system to cross-react with a zombie virus antigen, making her inadvertently immune. But isolating the cells that produce the antibody is a brutal process (at least, it was decades ago back when I learned about this stuff in class). In mice, you generally extract the spleen, separate the cells, and grow them isolated in culture. Then the cells are exposed to zombie antigens. Those cells that react will undergo changes that make them grow, while non-reacting cells do nothing.
The isolated colonies of cells that show reaction are then fused with cancer cells that make them "immortal". Normally, cells cannot divide more than a certain number of times before they become 'senescent' and die, but when fused with cancer cells that lack these restrictions they will grow indefinitely while retaining the ability to make antibodies. This would let them create a cell line that produces a "monoclonal antibody" that can be used as a treatment. But such a treatment has certain inflexibility that limits its use as the virus evolves in the wild.
But if the cells that make zombie antibodies are known, then they can be used to isolate the molecules that are being recognized by the immune system. Those antigenic molecules can be used to purify more antigen (possibly from the blood of zombies), which can be used as the basis of a vaccine. The old-fashioned techniques that I learned from the last century are slow and laborious, and new rapid techniques probably now exist to do this without taking decades of research by several teams of scientists with millions of dollars to spend.
Well, it's a comic book fantasy. I'm sure they'll have gallons of vaccine to work with before Issue 200.