If there are utility or (especially) high-voltage transmission line poles/towers in view, they will provide a reliable true vertical against which to judge gradients. I first noticed this with the pole lines parallel to the tracks between Tehachapi, CA, and the Loop when I lived in the area in 1964-5.
Of the bridges I have modeled and will be modeling in the future, deck girder, pony truss and concrete arch, every one is on a grade. The only level track on the layout is between the station start signals at the clerance points of the end-of-the-station turnouts. (Yes, those turnouts are part of the hyperbolic grade easements.)
That's what my prototype did. I see no reason to deviate.
Chuck (Modeling mountainous Central Japan in September, 1964)
Frankenstein Trestle: curved and on a grade.
https://www.google.com/maps/@44.155226,-71.365939,105m/data=!3m1!1e3
Willey Brook Trestle: on a grade
https://www.google.com/maps/@44.1986952,-71.4104041,105m/data=!3m1!1e3
And this bridge:
https://www.google.com/maps/@39.0959626,-84.5435791,552a,35y,357.92h/data=!3m1!1e3
The approach spans to this one are on a grade:
https://www.google.com/maps/@39.0898394,-84.5182307,262a,35y,313.29h,44.93t/data=!3m1!1e3
Also this ones approach spans: https://www.google.com/maps/@37.8468552,-87.6033466,563m/data=!3m1!1e3
This bridge appears to be on grade or is it. maybe its just the vanishing point. Who knows? maybe both. Yup its hard to tell. One fasinating bridge though.
I scratch built a bridge for my layout that is on a grade and is also on a curve. I am extremely pleased with the result and it looked design perfect once I put it in place. It's nice to get really lucky, once in a while.
Brent
"All of the world's problems are the result of the difference between how we think and how the world works."
Background scenery can be deceiving. There is a place in the local mountains called Gravity Hill. If you stop your car in the middle of the road and put your car in neutral your car appears to roll up hill, but it is an optical illusion caused by the hills surrounding you. Your car is actually rolling down hill but you can't tell because your perception is distorted, like those fun houses where billiard balls roll uphill into the upper most pocket of the table.
Also, don't forget that most prototypical grades are VERY small, i.e. 1-2%. Unless you have something level to compare against or a very long bridge, the grade might not be noticeable.
Even bridges on tracks that are paralleling rivers are on a grade: the slope of the land that defines downhill for the river. If the land were flat, you'd have a lake, instead. The Columbia River has a slope of .04%.
I would suppose there would be level bridges on routes that follow an ocean shoreline. And there certainly are lines like that.
Ed
Just out of curiosity, what did those bridges you saw have in common...if anything? For example, are you on the prairies? In a river valley with a slow-moving river?
I think you would have little disagreement if you were to claim that virtually no bridges are perfectly level. There are undoubtedly quite a few within 0.01% of level, and could be classed as level. But the heavy majority of bridges are on a grade, even if it's just 0.10%. In mountainous country where the right of way is making its way toward a pass, and it crosses several re-entrants or side-stream valleys, the chances of a level bridge across any streams or re-entrants is next to nil.
The reason is that rail tonnages require so much horsepower for each increase in grade of just 0.5% (it works out to just over three times the HP to get the same tonnage up a grade one-half a percent steeper trying to maintain the same speed) that they need long slight grades to clear mountain passes. Interruptions every 300 meters to cross a level bridge would necessitate even steeper grades between those bridges to make up for the loss of altitude gain over the decks of level bridges. So, rather than undulate and make the between-deck grade that much steeper, they try to maintain a constant gradient toward the pass.
I looked at Western Pacific Track Charts. On every bridge the track on the bridge was on the same grade as the track on either side. The only level (0%) grade on bridges were in areas where the track grade is 0%. There are even some cases (longer bridges or trestles) where the grade changes on the bridge.
In the Feather River Canyon the grade is 1% through the bridges.
When looking at bridges either in person or in photos we tend to see the bridges as level even when they aren't. Looking at the terrain is deceiving. A properly leveled camera would show the grade (if any) but most people taking photos do not bother, and many do not even have a way to level their cameras.
Highway bridges are often not level. There is no need for railroad bridges to be level either.
I tried to sell my two cents worth, but no one would give me a plug nickel for it.
I don't have a leg to stand on.
They might appear level, but many bridges are on grades. The MILW. had many bridges that were on a grade, and curved, at the same time. Engineers designing a railroad grade wouldn't interupt the grade just to make a "level" spot for a bridge.
Try doing a search for "railroad bridges on a grade", and you'll get lots of info, pictures, and links to various forums where this has been discussed. There was a thread in here, not too long ago, where it was discussed.
Mike.
My You Tube
Every railroad bridge i've seen in person and in pictures has been level. Are there any railroad bridges that have inclines / declines?