Comma between two independent clauses subordinate to another clause

Question

I have learned that I need a comma between two independent clauses, e.g.,

The tram service was disrupted, and the bus was too crowded.

What if the clause pair is subordinate to another clause, e.g.,

I went on foot because the tram service was disrupted, and the bus was too crowded.

Do I need the comma before and? I am inclined to avoid the comma so the last clause (i.e, the bus was too crowded) also appears as a reason for going on foot, not an independent clause. What is the more accurate punctuation, with or without a comma?

Answer 1

You need the comma; they are still separate clauses.

If you fear the impression that it's not the two reasons, perhaps you should recast the sentence.

Because the tram service was disrupted, and the bus was too crowded, I went on foot.