鉄道の道筋を決めるのは一体何だろうか。

railroad; railway; rail transport

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

path; route; itinerary

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to decide; to choose; to determine; to make up one's mind; to resolve; to set one's heart on; to settle; to arrange; to set; to appoint; to fix; to clinch (a victory); to decide (the outcome of a match); to persist in doing; to go through with; to always do; to have made a habit of; to take for granted; to assume; to dress up; to dress to kill; to dress to the nines; to carry out successfully (a move in sports, a pose in dance, etc.); to succeed in doing; to immobilize with a double-arm lock (in sumo, judo, etc.); to eat or drink something; to take illegal drugs

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

...the heck (e.g. "what the heck?"); ...in the world (e.g. "why in the world?"); ...on earth (e.g. "who on earth?"); one object; one body; unity; one form; one style; one Buddhist image (or carving, etc.); generally; in general

what; how many; you-know-what; that; whatsit; what's the expression; what do you call them; what?; hey!

be; is; indicates past or completed action; indicates light imperative

indicates a question (sometimes rhetorical); or; whether or not; some- (e.g. something, someone); indicates doubt, uncertainty, etc.; emphatic prefix; suffix forming adjectives or adverbs; in that way