5.5mm OD with 2.1mm ID pin size -arguably the most common size as many security cameras, power adapters for routers and so forth use this same size.
I know you didn't ask, but info to keep in your back pocket of train knowledge.
Fastrack is the funky one- they use the fatter 2.5mm pin size. So you cannot directly plug in a Z-1000 brick into fastrack because the fat pin in the connector of the Fastrack won't go into the smaller 2.1mm hole of the Z-brick connector.
Conversely, a Lionchief power supply for Fastrack from a starter set will plug into a TIU port, and because the smaller pin still makes contact, so you can use those Lionchief bricks, and repurpose them to power a TIU.