A good way to tell the type is to imagine the original sentence without a relative clause.
- A: 木村さんは犬を飼っている。 (木村さん is the subject)
- B: [私は]賭け事で財産をなくした。 (賭け事で is adverbial)
- C: (no original sentence; 翌年に太郎が東京へ行った would mean something different)
- D: (no original sentence; 事実 does not bite a baby)
And consider the phrase in question:
これまでとは対向線路にあたる闇の向こう
We can think of the original sentence, which is either:
- 闇の向こうはこれまでとは対向線路にあたる。
- 闇はこれまでとは対向線路にあたる。
The darkness corresponds to the oncoming lane (from the viewpoint of where we came from)
So 闇 serves as the subject of the verb あたる in the original sentence, just like 木村さん is the subject of 犬を飼っている.
This sentence may have confused you because the verb あたる is a verb that can denote an appositive relation using a verb. In type D, there is no such a verb.
- 東京に行った話 (Type D; the relative clause is the direct content of 話)
- 東京に行った気になる話 (Type B: You feel as if you went to Tokyo by the 話 (adverbial))
- 東京に行ったことと関係する話 (Type A; 関係する is the subject of 話)
In English, noun phrases within a relative clause cannot be further relativized, but this is allowed in some cases in Japanese.
Is this referring to nesting?
I think so. 頭が赤い魚を食べた猫 can both mean:
- a cat who ate a red-headed fish
- a fish-eating cat whose head is red
But if I'm not mistaken, "a cat who ate a fish whose head is red" can only mean the former (...am I right?)