That sounds like a good way to approach the situation.

Ask the other rider to fix their behavior. If they don't, ask the conductor to ask the other rider to fix their behavior. If they don't, I don't think you need to call the cops.

The conductor could just as much charge the rider some fee that can be posted in the rider policy of the train, in the train, in the ticketing app for the train, at the station, etc. This way the train owner/operator can collect some remedy for the rider's violation of the rules to help fund remediating the rider's impact to the train environment (like cleaning their specific seat when that's usually outside the conductor's responsibilities), including a markup if they want to add that.

Now, I don't think charging fees will work for every violation. But I still think it's a valid lever the train owner/operator could make use of instead of bringing the entire train to a halt.

If the rider refuses to pay up, then call the cops. Or, the conductor could also ask the rider in violation to exit the train at the next stop. That's another option. If that fails, call the cops.

Additionally, if cops are needed, the conductor could call them and ask them to meet the train at one of the stops coming up. This way the arrival of the cops could be timed with the arrival of the train to the station. Minimizes impact to everyone else's schedule, and ensures cops have access to the train since most of the time spent on a train is in between stations which often isn't accessible by road.

I think there are ways to better manage this than to stop a train as soon as a conductor witnesses a violation and start a train only after cops arrive to that specific location and escort the rider violator off the train.