Yes it would be an approximation because the game doesn’t know in which route you will put the train and the slope of the track. You can also change the track with time or even the slope with terraforming. Its an approximation.
I quote the Interactive Guide if I may:
You don’t need to worry about the train being stuck in the track as a result of adding too much cargo. The train will always have a minimum speed of 1 Km/h.
As the calculation of the maximum amount of wagons a train can pull depends on the weight of each wagon, the state of each wagon (loaded or empty), the gradient of the tracks in the route and the tractive effort and power of each locomotive, simple observe your train along its route. If its too slow consider changing to a more powerful one or adding more than one locomotive.
Remember in real life they don’t know that information either. They don’t buy a train knowing exactly the route it will be taking as a train is used for 60 years in different tasks. They don’t know exactly the slope of each centimeter of the track the train will run.
Sometimes they don’t even know the space available in the stations the train will use. 🙂 http://www.bbc.com/news/world-europe-27497727