Long train Bridge tension and compression angles (minimum brick count)
The compression member in your design looks thin to me, and the tension member oversize (assuming they are the top and bottom respectively). I would use string for tension and the technic beams for compression as the first step.

The low-part-count solution that occurs to me is going to kill the clearance under your bridge, but if that's acceptable a single column down from the centre with string/rope tension members to the end will work. You can use two columns with lateral bracing to get similar strength but better clearance, at the cost of needing more parts.



You miay also be able to do a modern single-pillar suspension design that looks exciting, especially if you're willing to cheat a bit with a non-Lego weight. You may be able to extend your beam and tie to that instead of using a weight, but I suspect you will struggle to get enough strength.



You can buy 100m rolls of string that look a lot like Lego thick string for next to nothing from sewing supply places. I have two, because the waxed string slides more easily over plastic for times when I can't use pulleys or rollers, but it's also less bendy so in low-tension situations often looks bad.