* Check passenger tickets
* Observe the cars & make sure people aren't breaking rules, etc
* Serve food/drink if applicable (e.g. for business class)
And so forth. This is how the KTX and Shinkansen work, and they know what they're doing.
Checking tickets can be done by not allowing anyone on the platform. Or world best practice is random checks and large fines if you are caught without a ticket.
Breaking rules is tricky. There is some need for that, though there are options, though in the end I will grant you this.
Trains should never serve food or drink. Passengers can get off the train at a station when they need that, and get on the next train. Space used for the food and drink is space that could be used for more seats. There is a reason all railroads have been trying to do away with food and drink service. It is considered bad practice everywhere.
> There is some need for that.
I see.
> Trains should never serve food or drink. Passengers can get off the train at a station.
There aren't going to be stations all across Nebraska or what have you. And even if there were, stopping repeatedly would defeat the purpose of high-speed rail. All that time speeding up and slowing down starts to add up very quickly.
It doesn't take long to unload and reload a modern train. France and the US do bad, but there are better examples where it can be done in a minute.