Learn Croatian: Osjećam neopisiv strah prije vožnje vlakom smrti. - I feel an indescribable fear before the rollercoaster ride.

It literally translates to train of death. The base phrase is vlak smrti, which is the standard Croatian term for a rollercoaster. The word smrti is in the genitive case (of death) and never changes its form when the main noun (vlak) changes case in a sentence.

Why is vlakom in the instrumental case instead of matching the genitive case of vožnje?

While the preposition prije (before) forces vožnje (ride) into the genitive case, vlakom acts as the means of transport. Just as you say you travel by train (putujem vlakom), the noun vožnja takes the instrumental case for the vehicle being used.

Since strah is the direct object of the verb, why doesn't the adjective neopisiv have an -og ending?

Masculine nouns in the accusative case have two different forms depending on whether they are alive or not. Since strah (fear) is an inanimate concept, its accusative form is identical to its nominative form. Therefore, the adjective neopisiv also stays in its nominative-looking form.

What type of noun is smrt and why does it become smrti here?