But it is both, and this has been covered by people a couple times.

I say both, but I also had no idea how much my language was bent by having a dyed in the wool New England Yankee around from childhood.

Down the pike from roadways, and down the pipe from aqueducts, and later pneumatic tube messenging systems

https://en.wikipedia.org/wiki/Pneumatic_tube

In fact, some of the people following up on this thought it more likely to be down the pipe "originally" or at least dominantly as pneumatic tube messages just show up without warning, whereas anything coming down the highway gives ample warning before arrival.

Language is general is fun. Hawaii and New England share lots of weird similarities that the rest of the US apparently does not. For me Aunt does not rhyme with Ant (that's not so uncommon because Auntie never rhymes with anty). And apparently this is the New England way as well. Tomahto is also not uncommon and that too is a New Englandism. Zoris too.