Somewhat different to today, where you would get the cheapest possible reinforced concrete design, and bugger what it looks like.
I think you'll find that even then they were going for the "cheapest possible" design and didn't really care what it looked like. The only reason old viaducts aren't made out of reinforced concrete is because they were built before it was invented.
Besides, who's to say that reinforced concrete viaducts won't be considered "beautiful" in 100 years time? I'm pretty sure many considered the brick constructions ugly and a blight on the landscape when they were new.
But yes, if modern planning regulations, AONBs, National Parks, NIMBYs, etc. all existed when the railways were being built originally, I think it's fairly safe to say that the vast majority of them wouldn't have been.
Two things really "made" this country; naval supremacy and an entrepreneurial, innovative engineering culture. Sadly, we've clearly lost both.