Speaking as someone who did, in fact, ride public transit in the 20th Century, before smartphones…
I read books and magazines. I knitted/crocheted. I studied my ground school materials to pass the tests for my private pilot’s license. I studied Irish Gaelic. I read newspapers and did crossword puzzles. I listened to music on a quaint device known as a “Walkman”. I talked to fellow commuters. I people watched. Sometimes I napped.
Most of the time, I found the time surprisingly productive.
Books. My reading rate has seriously decreased since I started riding/driving to work.
Books and weekly news magazines filled much of my time. So did gazing out the window and watching the landscape slowly progress through the seasons.
I still read books on the bus. I play with my phone or tablet only if I have finished my book, and have forgotten to bring another.
I read books. Oh, the tragedy of finishing the book in the morning & having nothing for the ride home. Or a bus driver on a pre-dawn run who preferred to keep the interior lights off. Kindle Paperwhite has solved that. And made it easier to read large books. In case that weighty history tome is too much after a hard day, there’s something lighter available.
People read newspapers & magazines, too. Or maybe talked to an acquaintance. Conversation with strangers? Nope. Nowadays, there might be a talker on light rail heading to the Texas Medical Center; he usually gets off at Ben Taub (the big public hospital) to get his meds renewed…
I had a longer suburban commute in the 90’s when I was in my 20’s and worked either tons of overtime or two jobs. In the mornings I did my makeup on the Metra on the way to work and in the evenings I slept. If I ever found myself awake and not needing to primp, I had whatever book I carried for reading at lunch.
Living in Chicago the last 16 years, can’t do makeup on the CTA - it’s too bumpy.
It was by no means the age before smartphones, but when I commuted on BART in the San Francisco area I’d more often then not have a book with me. I got quite good at reading it with one hand only when it was standing room only.
I ride Seattle area buses now, and have since discovered the joys of podcasts. So that is my main activity. I should probably try move back to books, since I almost always can sit down these days.
Cruise control: the reader’s friend.
We perfected the technique of staring blankly without directly looking at anyone. Then we’d think, daydream, or read.
Read (books, comics, magazines) or Music (walkman, CD player, MP3 player).
If I were riding with other people, we’d socialize, depending on how packed the bus/train was.
I remember the technological breakthrough when it was made possible to hear the other side of the cassette without having to take it out and flip it over. The future!
Hanging on for dear life as the driver tried not to let the bus drop below 55 mph. Didn’t happen all that often, but it sticks in your mind.
This. I only use the Red Line, and that’s every so often I have to go into the city. When I do, there are always several conversations going on among strangers.
I would look outside the window. That is how I learned my way around.
Note: I recently taught an 18 year old how to drive. He HAD NO IDEA where he was going! (Grew up staring at his cell phone or video games when being driven around.)
This is why kids should be given bikes and put on their own to get around by age 14 or so.
Or you could explain everything to someone from another time.
Book or newspaper, for me.
I’ve been taking public transit ever since 1985… of course, I was a wee lad for some of that time, so my entertainment choices were probably a library book or looking out the window. 
Definitely remember the joy of having my own walkman to listen to on the bus… listen to books on tape from the library, cassettes of radio music, store-bought tapes, later mix tapes made from CDs at home.
And in 1999 I got my first laptop, a heavy Compaq thing with maybe a 4 gig hard drive running windows 98… I don’t think I ever carried that when it was just commuting on the city bus, but I’d take it for the commuter bus to Toronto for career training classes.
Well, on buses in Halifax, Nova Scotia, during regular weekday rush hours, even a couple of years ago, it’s a regular gab-fest.
In Montreal it seems to depend on the route and time. And when I lived in Ottawa 20 years ago people used to talk so, no, people did and do talk on some public transit.
I remember hating getting on at certain times because if I was standing and holding onto a strap it was a lot harder to read.
I still read when I’m on a bus or train but now instead of carrying 2-3 books in my purse to ensure I never run out of things to read I’ve got several dozen magazines and a couple dozen books on my phone at all times. It’s a LOT lighter.