Learn Spanish (Latin America): Mientras subamos, escucharemos los rieles temblar. - While we go up, we will hear the rails shake.

With perception verbs like escuchar, using the infinitive means you perceive the entire action, or it focuses on the fact that the action happens ("we will hear the rails shake"). If we used the gerund temblando, it would emphasize an action already in progress at the exact moment we listen ("we will hear the rails shaking"). Both are perfectly correct here, just with a slight shift in focus.

Why is subamos in the subjunctive here instead of the indicative (subimos)?

Because the action hasn't happened yet! When mientras (while) refers to an anticipated future event—as shown by the future tense escucharemos—it requires the subjunctive. If you were talking about a habitual routine ("while we go up, we usually hear..."), you would use the indicative mientras subimos.

If I wanted to replace los rieles with a pronoun and say "we will hear them shake", where does the pronoun go?

You place the object pronoun before the conjugated perception verb: los escucharemos temblar. While attaching it to the end of the infinitive (escucharemos temblarlos) is grammatically possible, putting it before the main conjugated verb is much more natural and common.

Can I change the word order to escucharemos temblar los rieles?