You can tell you are spending way too much time at home when you start getting curious about how and why railroad things are designed the way they are. 
(Character flaw I guess) But I am a big fan of bridges in general, and particularly truss bridges. I have always liked the classic look of the Lionel 6-2122 ('76-'87) pictured below, though I've never found a picture of a prototype that Lionel might have based the design on. So I am not sure how realistic the design really is; it could be a Lionel "fantasy" design for all we know. The bridge is 24" long, or 96 scale feet end to end, single track.
But I thought it might be fun to simulate the member loadings, based on what if an actual bridge were to be made this way, using semi-real railroad weight loadings. I found a cool little truss design simulator on the web put out by JHU (John Hopkins University) Engineering. Neat little package with a simple interface that works well once you have spent some time with it. There are no instructions as such, so you are kind of on your own. Easy to Google if you are interested.
The 3 basic truss bridge designs used are the Howe, Warren, and Pratt trusses. For case 1 below I first did a basic Warren design of the bridge, because it most closely resembles this. It uses the same dimensions and node locations as the 2122, but lacks the vertical members used in the actual model of the bridge. I used loads of 70 tons per support node, which is loosely based on the 500 ton weight of a Bigboy, reduced proportionally to the scale 96' length that would be on the bridge at any time. So the total load supported by the bridge in this scenario is 96/126*500/3/2 + 10%. The 3 is because there are 3 load points; the 2 is because we are looking at one truss side only. The 10% is a GEP safety factor. So here it is:
You will note that all end and top members are in compression loading, all with significant loads. All others are in tension loading, except the 3rd diagonal members from each end which are in minor compression loading. Typical Warren truss loads. Also note the large loads on the end and top members, particularly the two end members and the top center member. This helps explain why the entrance portals on truss bridges are heavily gusseted and reinforced.
For case 2 below, I added the extra vertical members exactly like the 2122 model has, and re-distributed the loads, just to see what effect this would have on the various loads. Note the total load is the same as case 1. So here it is:
Notably all the same members that were in compression in case 1 are still in compression in case 2. All the rest are in tension including all the new verticals. The loads on the entrance portals remains the same, but the loads on all the other top members are considerably less. The 3rd diagonal member from each end is still in compression, though the load is now more than double that of the same members in case 1. Still not that high though. It also shows that several of the diagonal and vertical members used in the model could have been made much smaller, if the bridge existed in real life. Notably the members between nodes 8-9 and 5-13 are 18" fabbed beam on the model, but they could have been reduced to 12" easily. Also the diagonals between nodes 7-2 and 6-2 could have been 12", or even smaller, instead of 14".
OK, so what? Well nothing really; this was just for fun and interest sake. But I conclude the 2122 model is probably not based on a real prototypical design, because the sizes of some members on the model just don't make sense. It also shows that the added vertical members are not really needed, because the bridge design is just fine as a basic Warren truss per case 1. And case 1 would be way cheaper to build. But it's still a great looking bridge model IMO.
I would be interested in any comments, but please understand that this is just for fun and based on lots of assumptions and guesses!
Rod
