Seatposts...
 

My folding bike (heavily modified Apollo) has a seatpost that simply folds by being pushed down into the frame after undoing the quick-release. I may be using the bike to commute over the summer, but I'll be taking the train for part of my journey, so I'll have to fold the bike to take it on the train. Will being lowered and raised twice a day cause undue wear on the seatpost or the inside of the seat tube? They're both steel.

I'm sure you'll see wear eventually. But eventually is far down the line, probably after the bike is junked anyway. As long as the action is smooth you've nothing to fear. Just be sure the QR is adjusted so you're not forcing it while partly clamped. If there's scraping or friction when it telescopes have a local shop ream or hone the frame's seat tube smooth.

Brompton uses a plastic bushing so to be around the seat post,
it snaps in, and is glued into the frame..

Apollo is a foreign brand, here, [no clue as to what you have]
and American's don't care or learn much about history anyhow..
ancient Greek or otherwise..