1 PF train motor manages to pull my double Horizon Express when fully charge, and it does so at a decent speed. It's quite surprising because of the weight distribution on the HE train and simply because of the weight of the double train itself. Nevertheless, I still run mine on a double motor because I don't want to overwork the motor.
In my opinion, the bottom line is - there's no objective value. Of course this concerns solely large models. Everything depends on the weight of the carriages, the setup of the train, the layout you're using. There are a lot of factors to consider. But as a rule of thumb I wouldn't recommend anymore than 3 passenger cars on 1 engine, just to keep your parts safe.