The gross million tons calculation for railway tracks involves multiple factors and is a complex process, often determined through detailed engineering analysis and simulation. Here are some key factors considered:

The specific method for calculating gross million tons would likely involve mathematical modeling and simulation, considering these factors simultaneously. It's common for railway engineers to use specialized software for track analysis, taking into account the dynamic interaction between trains and the track structure.

For accurate calculations tailored to a specific railway network, consulting with railway engineering experts or utilizing specialized software tools designed for track analysis is recommended. It involves detailed data collection, analysis, and application of engineering principles to ensure the track's long-term viability under the given operational conditions.

Whilst I understand that you want something better than rules of thumb, the previous answer demonstrates that this can get very complicated, very quickly. Understanding some of the key relationships (I think axle load impacts with the cube, but train speed with the square) can be helpful, otherwise it gets unmanageable - you end up with more categories than metres of track! Some railway infrastructure organisations (e.g. Network Rail in Britain) therefore categorise track maintenance requirements by simple cross-tabulations e.g. of maximum speed and total tonnage per year, although one might reasonably treat track on structures differently from that at grade. Lastly, being raised on an embankment is another category you might want to consider.

@mr khalak .. please share some calcultaion methods