[Q]

snip

You may be combining two sets of terms - girder vs. truss, and deck vs. through. Girder bridges are simpler to build and maintain, but have are limited in length between supports. truss bridges can span much greater distances, but are more complex and more costly. For any given span and load, a deck bridge will be lower cost than a through truss (the main bridge elements are located closer together (under the rails or nearly so) so the lateral members can be smaller. Also, the deck bridge does not impose any clearance restrictions on the loads the RR cars can carry, while especial through truss bridges and limit heights and widths of loads. However, deck bridges use height under the track, and thus require the tracks to be at a greater height to provide clearance for the highway, railroad or waterway passing beneath. To lessen the grades for the approach to a bridge, you often see deck bridge spans on the approach to the through span which crosses over highway or RR or navigable channel of a waterway.

One of the best sources I've found for prototype practices in abridged terms for a model RRers' use with getting too bogged down in civil engineering is the NMRA Data Sheets. Join the NMRA and get a copy of these valuable documents. See

Gary Q

A good source of information is a Yahoo Group devoted to the prototype your interested in. Just go to

and put it the prototype you seek info on. Then join the groups that fit and enjoy the drink from the fire hose!

Peter

"Thomas M. Sasala" wrote in message news:CR_0e.6018$%d7.2400@lakeread03...

*** My other question is, aside from this newsgroup and the variety of BBs, are there any good websites or books that cover matching prototypical behavior on a model RR? I goal is to keep the hobby fun and not get bogged down with a lot of details, but I'm having a hard finding basic information like the best height for a tunnel opening, width of road ways, etc. Any suggestions would be great. Thanks.

----------------------------------------------------- Two good books that will prove helpful:

"Track Planning for Realistic Model Railroading" by John Armstrong:

"Model Railroad Bridges and Trestles":

These books are up to 32% off list price and include free shipping on orders over $25.

Bill Bill's Railroad Empire N Scale Model Railroad:

History of N Scale: Railroad Bookstore: to 1,000 sites:

A truss bridge is one with intersecting members forming Xs and triangles. Girder or beam bridges just have solid sides. So, I think you're really asking why use a through bridge, where the train goes through it, rather than a deck bridge, with the train running on top.

I'd say cost is the deciding factor. A deck bridge takes less material, since it doesn't have to go all the way around the train, so is preferred. The bridge may have to clear things underneath, like roads or waterways with boat traffic. If clearance is an issue there will need to be a through bridge.

A high bridge over a gorge would most likely be all deck bridge. A bridge over a river may have deck spans on the ends, but through spans over the main channel. Take a look at your bridges again to see if you can figure out what is under the deck and through parts. There should be some reason for it.

Here's a site on building a truss bridge from file folder material that you might find interesting:

The NMRA website has info on clearances, that would give you dimensions for a minimum tunnel portal and overhead bridge height. I believe there are engineering standards for roadway widths, but I don't know of any websites with the info. Think cheap. Why would anyone build a tunnel or bridge bigger than necessary?

Thanks for the input and correction of my terminology.

I've been to the NMRA website and their information is=20 sort of hard to follow. Or at least hard for me to=20 follow. Once I have a good look at it, I suspect I=20 will have more questions.

Thanks again.