There’s no point saying how much weight a train can pull, because it can pull many different weights at different max speeds, accelerations, etc…
What would be useful is if, as you added wagons to an engine, it would give you feedback on what the current maximum speed and acceleration will be, on flat, moderate, and heavy uphill inclines. Even that I’m sure would need to be an approximation, but it would be good to get a general idea of how a train will perform at a glance.