Hi All:
I'm currently building my new railroad which will have a lowboy trestle and a small plate girder bridge, both over water. The question is, is there any hard and fast rules regarding bridge track? I've looked at pictures on-line and some bridges have the extra inner track and some don't. What is the norm?
Ian
There was a discussion about guard rails on bridges earlier:
http://cs.trains.com/mrr/f/11/p/169494/1861783.aspx
It looks like it "just depends".
Ed
Bridge ties are thick because there's no cushion of ballast between them and some rather unyielding steel.
If you're following a prototype, then follow your prototype. Some had guardrails on everything, some only on bridges that exceeded a certain height or crossed things (like fields of pointy boulders or lots full of new Ferraris) that wouldn't make good landing places for falling freight cars. Others only put guard rails on through bridges - to protect the bridge, not the rolling stock. Others don't use guard rails at all.
Present day practice seems to favor ballasted deck bridges, and the rail/tie/ballast combination avoids bridge ties and is treated just as if it was on solid ground. Even then, there might be guard rails - again, to protect the bridge structure or whatever is underneath it.
Note that if guard rails are installed, they usually extend about 40 feet beyond the bridge itself, with the final 3/4 or so tapering to a point.
Chuck (Modeling Central Japan in September, 1964, with lots of plate girder bridges and lots of guardrails)
I want to clarify a few things. Bridge ties are not usually square, except on timber trestles. On open deck steel bridges they are much bigger, since the stringers (usually 2 or 4) or girders are rarely directly under the running rails. The bridge ties have to be robust enough to transfer the train load to the supporting members. On CPR they are typically 10" wide, and the depth will depend on the layout of the above mentioned steel floor system that supports them, usually between 12" and 16".
Ballasted deck bridges have many advantages, but increase the dead load due to their greater weight. Sometimes an existing structure can handle the extra weight and the railroad will replace an open deck with a ballasted deck on the original structure. Other bridges, especially through truss spans, would require too much upgrading and the open deck is retained.
The guard rails are often called Jordan rails. My understanding is that a Mr. Jordan invented the idea, patented it, and collected royalties. Some railroads felt that the 4"x8" spacer timbers on the outside of the deck provided equivalent protection, although that is debatable. But they would avoid paying any royalty. I have also seen other variants, such as three straight rails inside the gauge, with no taper. Presumably that also avoided using Mr. Jordan's design. Where the bridge carried multiple tracks, often only the outside tracks had guard rails.
As Chuck says, take a look at the prototype you are trying to follow.
John