I was thinking something along those lines, when you said private networks, do you mean VPNs? I also thought they might just be connected to a local server at the station itself, and that server is connected to the internet (on a private network).

0xbitcoiner

Ticket machines generally aren't on the internet but use private networks. The payment terminal often is (nowadays) but these 

 be isolated due to PCI/DSS with only a limited serial / USB connection for control flow.

[ vending machine ] --lan-- [ station control ] --wan or VPN-- [ backoffice ] 
     |
     | (control only)
     |
[ terminal ] ---- [internet or local network payment gateway]

Windows 2000 running a rail ticket machine in Portugal

Ticket machines generally aren't on the internet but use private networks. The payment terminal often is (nowadays) but these ***must*** be isolated due to PCI/DSS with only a limited serial / USB connection for control flow.

like so:

```
[ vending machine ] --lan-- [ station control ] --wan or VPN-- [ backoffice ] 
     |
     | (control only)
     |
[ terminal ] ---- [internet or local network payment gateway]
```