Just take a look at your bridge for Map Pack 1 Level 16, and think about what made it difficult. It took me over an hour, even though I already had plenty of practice with arches. It's difficult because the bridge is so large that it sways and must be very carefully cabled and reinforced. I had one verson of MPACK116 that actually stood up with no broken links after four passes 80% of the time (it depended on when you started the train), but it would start swaying and tear itself apart if you just let it stand there for ten minutes without even running the train.
If you could make the tension on cables tighter, it would make things ridiculously easy. After I completed all 48 levels that come with Pontifex, I started going back and making them cheaper. When you're cutting costs, even smaller bridges can use a lot of cable, and the challenge is to make stable designs that don't oscillate and tear themselves apart--a challenge imposed mostly by cable slack.
If you could increase the cable tension, this aspect of the game would be totally removed, and almost every level would be much less challenging. Without slack, the bridge doesn't oscillate, and it quickly stabilizes itself after the train passes, so there would really be no point in levels requiring four train passes--you would know immediately as soon as the train made it across once that every other run would be successful because the bridge wouldn't be swaying.
OTOH, a good addition might be high-tension cable at a higher price. It would have to be at least three times as much as regular cable, though.
