Long train Bridge tension and compression angles (minimum brick count) Hello master builders.

I'm working on my first large project (and first Lego stack overflow question).
I'm making a bridge that needs to be 3 feet long. Unfortunately I don't own tons of Lego bricks. 

Working with what I can find, I've built a single supporting brace of #8 rods and #4 angles. (I have the most of these in my bricks tub). Along the top (where the train track sits) are plates. Each platform connecting the plates is a 16 stud gap. The platforms are 4x4 studs.

This doesn't seem to work out the same when trying to pin the connecting beams on the bottom. Below is an early photo, I've reinforced the top to support the weight of my train, but it still bows slightly because the holes in the Technic beams do not seem to line up to the gap length. I think it's off by some percentage of a hole. (That makes me question if the top is actually an even 20 studs between the connectors)



The new design is to have more beams on the bottom and use angled lift arms to shorten the distance by some percentage between the connectors.

The question is, what angle lift arms should I be looking for? And how many variations of lift arms might work. Assume I have enough #4 and #5 rods to pin it all together.

To note, I know that a balance beam for a bridge is probably not a great plan, but I didn't find enough pieces to make two beams across my 3 foot gap.
Other suggestions for bridge construction are welcome, but I did search around a bit and either lack the right bricks or am too high off the ground to build support columns (5+ feet high)

Thank you for thinking on this and reading.
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.