@rosactrl #openTTD is an open source implementation of #ChrisSawyer's #TransportTycoonDeluxe: https://www.openttd.org/
I'd say it's more about transporting goods to grow industries than public transit though, even though it does have passenger and mail transport. #SimuTrans is a similar game: https://en.wikipedia.org/wiki/Simutrans
Then there's the #CitiesInMotion series, which has a stronger focus on public transport: https://en.wikipedia.org/wiki/Cities_in_Motion_2
A while ago I watched a (German) documentary about #SubwaySim Hamburg, which looked quite interesting, but apparently the game is not out yet: https://store.steampowered.com/app/1925090/SubwaySim_Hamburg/

#MiniMetro is a highly addictive subway simulator: https://dinopoloclub.com/games/mini-metro/
Though perhaps that one is too abstract?

#CitiesSkylines has quite a decent public transport system too, from what I remember, but I feel like its main focus is still on building a city in general, with the public transport just one aspect of that: https://en.wikipedia.org/wiki/Cities:_Skylines