Here's why:
* Prepositional phrases start with a preposition (like "for", "on", "in", "with", etc.) and end with a noun or pronoun (the object of the preposition).
* In "for laying track", "for" is not followed by a noun or pronoun. "Laying track" is a gerund phrase (a verb form ending in "-ing" that acts as a noun).
Therefore, "for laying track" is a gerund phrase acting as an adverbial phrase modifying the verb "laying".
It tells why someone is laying track, answering the question "For what purpose?"