“The Alcos’ smoke was visible for a mile”

By the use of the apostrophe here you are implying more than one Alco in the consist?

A better example, and one that confuses only me, I am sure, is the use of the apostrophe in "its" vs. "it's". The possessive or the contraction, which one gets the '?

Well, I'll take a stab at an answer..............

Alcos' would be a plural possessive form - i.e. the smoke from more than one Alco. Which reminds me of a rule I learned from an English teacher eons ago: Inanimate objects (as opposed to living entities) cannot possess something. So that means Alcos', and even Alco's, (the singular possessive) would be incorrect usage. Better to write, The smoke from the Alco, or The smoke from two Alcos (plural, which needs no apostrophe).

The word its (no apostrophe) is an oddity in that it means the possession of it, which would ordinarily require the apostrophe. Go figure! The contraction of it is, is it's, and once again, we run into that above rule - can an it possess anything?

Now - all clear as mud??