Seems that it would be simple to just measure the ground to
the 'floor' of your box cars, they should all be the same. That would be the top height of your loading platforms.
If you are asking about height of your layout table
I would suggest that you sit on the floor in the position
you would be working under the table, measure to the
top of your head and add a couple inches. That would
be the best table level for you. You could do what needs
done without bonking your head.
In the U.S.: Ground level platforms are 8” above top of rail. (Or level). Some commuter lines use low level platforms, 25” above TOR.
High level platforms in the East are 48” above TOR.
Subways and rapid transit will vary.