They sometimes have agreements to allow transfers between them, like the two Tokyo subways. That's a pretty gross understatement. There are more than a dozen

They sometimes have agreements to allow transfers between them, like the two Tokyo subways. That's a pretty gross understatement. There are more than a dozen companies operating public transport systems in Tokyo alone and it's all interconnected. They don't only seemlessly interconnect (via PASMO or SWICA smart cards), but they also share common designators for their station labellings. So if you are going to use two different subway systems in Tokyo, you buy a transfer ticket and have to pass through a gate in the middle of your trip to transfer to the other system. I remeber exactly that. But that was around the year 2000, where it was really difficult to use public transport without a guide as a foreigner. Today, with a smart card, consistent and unique labelling of every station, regardless who operates it, and all ticket machines talking perfect English it's a breeze.