How are the multiplayer opponents placing multiple girders in one turn

N.O.V.A. should be saving your progress automatically, and there should be a continue option at the top of your menu the next time your game starts.

If it doesn't, your best bet is to re-install the game, as this usually seems to work for others who had the same problem.

Yes, that one is difficult.

You just want a hint, so it's hard to say exactly how to provide a good answer to your question. But here goes.

Hint #1

My solution involves first getting 4 Green and 4 Purples, not pre-merging the primary colors.

Hint #2

Split them up so the Greens go to one side and the Purples go to the other.

Hint #3

Focus on either Green or Purple side for your solution. the other side can be a mirror image (or rotation) once you work out a solution for the other side. You only need half the map.

Hint #4

My solution involves 3 merges each: 2 on the top merge, then 2 on the bottom merge, then the results merge.

Hint #5

Your first 2 merging stations (to go from 4 to 3 and 3 to 2) should only take 4 tiles total - a small loop that goes away from and then comes back to a track. This is because the trains are only 2 tiles apart.

Hint #6

All 3 of my merges on each side are within 1 time increment of one another (a time increment defined as how long it takes 1 train to move 1 tile).

Hint #7

I'm running out of hints here without showing you a picture.