Huge assumption, there is a wide range of various parameters that goes into how accurate you need an response to be, depending on context. As sure as there exists questions that you need 100% accurate response regardless of response times, I'm sure there exists questions on the other extreme.

In this situation you would have someone with actual knowledge of the mechanics involved do the computation using the actual data (e.g., what's the mass of the train? Which kind of breaks does it have?) instead of asking an LLM and trusting it to give the correct answer without checking.

Assuming you could find an expert like that in time, and that they will then be able to understand and solve the problem fast enough to still be helpful.

If you need the answer within a couple hours, you can probably get it for an expert; if you need to get an actionable answer within minutes, based on some back-of-the-envelope calculations, then a SOTA LLM is a much safer bet than flagging whoever seems the smartest in the room and asking them for help.

I assumed we already did such calculations in advance, as it's needed to have proper safety measures.

Why? Lets say your are designing a railway system. It does not matter if it takes 1 sec or an hour if the planning process are months long.