Learn English – “Obstacles along the road to success” vs. “obstacles across the road to success”

I fear your categorization is based on unsound foundations, but let's just look at along:

The signs were on buildings along Main Street.

Passive.

He traveled along roads that were long and painted purple.

Active.

Assuming that I correctly understand how you're categorizing prepositions, then it's a preposition that can be in either state.

There are no special rules for rarely used prepositions. According to the Chicago Manual of Style, all prepositions should be lowercased (except when they are stressed or occur as the first or last word). But I hardly ever see this style used. The AP style is much more common. According to the AP stylebook all prepostions containing three letters or fewer should be lowercased, while the Wikipedia Manual of Style says all prepositions containing four letters or fewer should be lowercased.

However, the Wikipedia rules are not enforced, and as a result the capitalization on Wikipedia is a mess, and often in violation of the style manual (as you had observed). I think the reason for this is that the AP rules are dominant in real life, so this is what people are used to (i.e., what “looks right”). This explains many of the cases that you listed: down, over, unto, upon etc. are all capitalized according to AP style.

So regarding your examples, it depends on the style guide you choose whether the four-letter prepositions should be lowercased. Three- or two-letter prepositions like on, off, out or up should always be lowercased (if they are really used as prepositions and do not occur as first or last word). Yet should also be lowercased, if used as a conjunction.

There is no difference between book titles and song titles. The AP style for instance says that its composition title guidelines apply to book titles, movie titles, opera titles, album and song titles, radio and television program titles, and several more.