Learn Dutch: We wachten op het perron. - We are waiting on the platform.

You use welk. Because perron is a het word (as we see in het perron in this sentence), the word for "which" is welk. If it were a de word, or plural, you would use welke.

Why does the Dutch sentence just say wachten instead of translating "are waiting"?

Dutch doesn't usually use a separate "are doing" tense the way English does. The simple present verb wachten covers both "wait" and "are waiting".

Could I use wij instead of we here?

Yes! Wij and we both mean "we". We is the standard, unstressed version that you will hear most of the time. You would use wij if you wanted to emphasize that we are waiting, as opposed to someone else.

Does the preposition op always mean "on"?

In this context, yes. Just like in English, you wait "on" a platform, so you use op. Prepositions do not always match up perfectly between languages, but here it is a direct translation.