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?