At which point does a phrase start to become parallel? For instance, in this correctly written sentence:

Just as many commuters would take the train as would take the bus.

The parallelism begins with the word “would”. But couldn’t it begin in other places such as “take” or “commuters”? Is there a rule?

Just as many commuters would take the train as take the bus.
Or
Just as many commuters would take the train as commuters would take the bus.

If there is a rule, I don’t know it. Intuitively, though, it feels clunky to repeat the noun. I don’t want to see “commuters” twice. The first two are fine, and I would even say that you could trim it further and say, “Just as many commuters would take the train as the bus.”

In a straightforward comparison like this, if you’re aware of the concept of parallelism, then you can probably trust your ear and be fine. It gets trickier if you change the meaning of the sentence slightly:

Just as many commuters would take the train (if one were built) as currently do take the bus.

That’s parallel, but the verbs are necessary because they’re different tenses.