The problem with going down at that location is that there is only the 4 - O42 curves to achieve the drop of 5" is a 9.5% grade to provide clearance for train, then you have increase that another 3.75" to clear the framing.
Since the train is almost 14' long, this needs a rethink.
I just tried a few different ways. Not enough room to stay at level. From the switch near the bridge to the switch at the top of the O54 loop, it will hold the engines and six cars. That's both 90° corners and the long straight run. That's a 5.6% slope in 143" to 8".