What do you think were the most important milestones in Maine's pre-Civil War "industrial transformation"?
In my opinion, there are three important milestones in Maine's pre-Civil War industrial transformation: Maine's secession from Massachusetts, Maine's port (well-known to be "ice free"), and Maine's rail link (which does postdate your question a bit because it became significant just a few years after the Civil War).
We have to talk about Maine's secession from Massachusetts because that is what helped main become its own industrial entity. Until 1820, Maine was nonexistent and considered part of Massachusetts. As a result, Massachusetts would have gotten all of the credit for the rapid industrialization there. There was one vote earlier on that failed, but the vote succeeded in 1820 and made Maine a part of the Missouri Compromise. This had the added bonus of equalizing the number of slave states and free states. Of course, Maine was a free state.
Of course, when one considers actual industrialization, one must consider transportation in regards to Maine. Maine has a major sea port which was amazingly "ice free." As a result of this, it was the main port near Canada! This meant, of course, that even in the harshest winters, industrial goods could be shipped in and out of that port in Maine.
Maine ports play a key role in national transportation. Beginning around 1880, Portland's rail link and ice-free port made it Canada's principal winter port, until the aggressive development of Halifax, Nova Scotia, in the mid-1900s.
Then, just a few years later, Maine had it's own rail link. The addition to railroading to Maine's vast shipping advantage made shipping goods in and out of Maine even easier! Suddenly goods could be shipped not only by water but also by rail. Therefore, I would consider Maine's port as well as the addition of Maine's significant rail line to be the most important milestones in Maine's "industrial transformation."