It’s exhausting.
On a per-vehicle basis, buses are cheaper to run than trains. On a passenger km basis, if both vehicles are full, the train is cheaper to run than the bus, keeping capacity constant.
Buses are great for low-demand routes. Great transit systems use buses extensively for local routes, solving that last mile problem at a reasonable cost, cheaper than taxis can.
Trains are great for high-demand routes. If you have a dense urban core with lots of demand for mobility, trolleys often provide a lot of service for relatively fast speeds which is faster than walking. Trains are great when you have long distances with few stops where lots of people are going to be traveling between two places. There are always going to be fixed routes with predictable high demand in larger cities. Even smaller cities of around 100,000 people can often have routes with higher demand where a modern trolley can be the most optimal option. Likewise, using a trolley for a long distance high demand route is generally foolish, and you would be better off with a faster, higher capacity train like the NYC Subway or BART.
It’s all about what is most optimal for that route in order to build a network where trains and buses work together seemlessly.
The most useful transit systems typically use a combination of trains, trolleys, and buses. Trains serve the high demand predictable fixed routes, buses provide local connectivity, and trolleys in dense urban cores provide faster high capacity transportation for those short high demand routes.
That is how you build a modern transit system which people love to use.