Here's a six minute video of load testing of new bridges on a new German high-speed line -- with a ballast train.
Yes, the narration is in German, but you can use Youtube's auto-translation -- or just watch the images.
Yes, there's a reason why they used two single-track bridges over this valley. There are tunnels on both sides of the bridges, and with current European and German safety regulations, you cannot build new long double-track railroad tunnels. Because the bores have to spaced apart for safety reason, that meant that the bridges also had to be some distance apart.