Trains need lots of cars to be filled to profit. So when you have a bus, most likely it’s filling every time. But with a train, it’s not going to fill unless the station is positioned well OR you have feeder bus lines within the city. Train lines tend to be outside the city and you’ll want it near the part of the city (if it’s nice and separated) that has residents or industry, depending on what you’re doing, but a bus line to the other segments is key too.
From one city to another next to it, you’re going to end up with a break even point of X passengers per train. You want to be able to carry twice that amount. The trip is going to take a certain amount of time, and likely if you’re past 1900 you have a fast train and will only need one train for the route. If you’re going to 3 or 4 cities you might want 3 trains to run it. Regardless, you want your frequency to be low enough that you have probably at least 5-8 minutes to walk/take buses/etc to get to and go from the station. So if you have a 12 minute round trip you need to run at least 2 trains, because the round trip + the freq is how long your passenger will be waiting on + riding the train max. So if a train arrives every 6 minutes, and travels to the other town in 6 minutes, you have a 12-minute wait for the train. Less wait is better but less wait costs a lot of money, ie, the entire cost of a train, and reduces the per-train amounts of things being shipped, even though more people might take your lines. Keep in mind also that you have to adjust your buses so that you have enough that they can drop enough people off within the time frame to fill your train. If not you’re not going to be able to fill the trains even if people want to ride.
As for cargo, cargo isn’t like people. You build a station by the raw material. You build a station by the goods producer. You need that goods producer to be making a ton of goods. The most important thing to remember for the line between the two is the 20 minute rule. As long as your freq with 1 train is under say 18 minutes you’re golden. You don’t even need a second train, which means less maintenance costs, but a longer train. That’s a good thing. Lots of cars. Start with them so you don’t have to interrupt service. The goods will wait for you to pick them up.
That sets up the one chain, the other side of the chain I haven’t really mastered yet, to be honest. But I think what needs to happen is bulkifying the train route. You want a route that involves a train servicing several towns, because goods don’t bulk up like the raw materials do. So to get enough to be profitable, you need to run a long train that serves several areas, and then have trucks distribute out from there, or have the one train make several stops (but then it’s running half-full some of the time) and there’s no good back haul either.
But I know it’s possible, at least it was on moderate, but I bet it’s possible on hard too, you just need more bulk and/or more distance between the original station and the target areas.