I would recommend 7 inches or 17.78cm from the lower track railhead to the bottom of the overpass wood.
You can have less if it is not available. Measure the height of the tallest engine or car that you have. That will be the limit.
Double stacked container cars and auto rack (carrier) cars tend to be the tallest.
I am sorry that I didn't comment on the trestles - I had a loss in the family and the holidays were a bit tough. The result looks really great!