My upcoming layout will include a portion of a loop crossing about 4" over a lower track. I plan to keep the opening (between hills) about 18" wide, with straight track, so I can use a bridge kit of some sort (I porefer not to build from scratch). I think I can adjust the connecting curves so I could vary the width of the overhead track somewhat, perhaps from 15" to 22". The connecting tracks will on 5/8" plywood with cork roadbed and Atlas code 83 flextrack.
a) what bridge kits / support piers do you suggest looking at? I see a Micro Engineering girder bridge setup, for instance.
b) please suggest how to handle roadbed and track across the span. Should I just continue the cork and flextrack, or switch to bridge track (can I splice pieces to get the correct length). If fancy track, how do I handle the roadbed…or just put the track directly on the bridge? How is the transition best made from the plywood to the bridge…router a notch in the plywood end and rest the bridge atop the notch?
Either a through truss or a through girder would apply in your case. There are quite a few available on the market, some from USA and some from Europe. I have the combination 150’ through girder combo from Micro Engineering. It takes a bit to get it assembled nice and square, or it did in my inexperienced hands, but I managed.
In the case of Micro Engineering, I believe I ordered enough pieces of their bridge Code 70 rails to cover the span. They just sit on the components. Or, you can fashion your own by nipping all the webbing between the ties on flextrack and scrunching the ties together. It is hard to do that to the fixed ends of the ties.
LION used Atlas chord bridges on his last layout. Each bridge has two 9" lengths of track and so is 18" long.
LION used two bridges end to end. He used sheet metal and a brake to bend two lengths of support 36" long. This supported the span without a center pier, which would have been un-prototypical but necessary since it would of required the pier to be in the middle of the track below. (Poetic license applied for.)
If you want to use the smaller girder bridges, me thinks they are only nine inches long, but two of these end to end would give you the span you need. I do not suppose that you happen to have a a sheet metal shop with brake and cutter handy, perhaps some plastic beams of some sort glued to the bottom to the girder bridge will work. So will a piece of wood, just make it narrower than the bridge so that it will not be seen, or else can be disguised as a steel structural member.
On my new layout the two Atlas chord bridges are side by side, with a sheet of hardboard in between them since this is a four track main line. Eventually I will finish the bridge with structural members between the two bridges to make it look like a single four track bridge.
When building a bridge is necessary, a railroad will use the least expensive alternative. They would not span an un-navigable brook, a single track and a country road with a single long span. They would use at least two - and more likely three if piers can be built to support them.
Deck spans are preferred to through spans - no interference with oversize loads. Through trusses and loaded Schnabel cars don’t mix.
While slightly more expensive to build, ballasted decks are preferred - track maintenance is much easier, and no specialized timberwork is required. Many railroads don’t even bother with guard rails on ballasted decks.
To model short span ballasted deck through/deck girders, just square up the edges of the subgrade and apply the girders from an Atlas bridge kit, modified as necessary, to the edge.
Railroads use skewed bridges - rhomboid rather than rectangular footprint - and align the abutments parallel to the rails or road underneath. Saves $$$ by reducing the amount of steel in the bridge, especially if it’s a through truss.
Some railroads had a `standard’ deck girder that was used just about everywhere - they even kept them, preassembled, in the MOW yard along with panel track and other emergency repair supplies.
If you’re modeling my prototype, bridges come in two colors. Older ones are oxide red. More recently, the finish of choice seems to be baby blue! (Thank God my bridges are all old!)
Chuck (Modeling Central Japan in September, 1964 - with oxide red deck girder bridges)
Chuck makes an important point. Develop the rationale for your bridge and it will help guide you to an appropriate solution. It’s not just the length to be spanned. Take into consideration the budget such a bridge might have it it were a real-life situation, the engineering standards such a line might have, etc.
Another option is to use trestlework for the approaches, covering most of the 150’, reserving a clear steel span just for where it crosses over the line or roads, etc underneath it.
My favorite bridge, or rather, the one that I want to sort of model is the crossing of the Gowanus Canal by the IND at Smith-9th Street. Here is a photo that Fred Gunther took from a canoe:
It is a FIVE track subway crossing, the station is actually on the bridge (two side platforms) so figure that the bridge is seven tracks wide. It is a very tall structure, you can see that the drawbridge protecting the street fits entirely under the railroad bridge. The tracks were slapped down right across the tops of existing four story buildings:
It is the only part of the original IND that was built upon a structure, the rest is all subway, and on wonders why the made a bridge over the canal instead of a tunnel under the canal since they had no trouble tunneling under the East River (three times). [I am told that there be MONSTERS living in the muck under the Gowanus Canal.]
Here is what I am building.
The gray board and all that is upon it has been replaced with the Canal Street Station, the Gowanus Canal will be built above that, and there will be appropriate stuff along 9th Street, including buildings under the structure.
The muck under the Gowanus Canal IS the monster. It’s so (comparatively) thin and runny that early 20th century tunneling methods couldn’t handle it.
If the same job was contemplated today, it would be done using the following techniques:
Sink a pre-assembled tunnel structure into the mud. Suck out the muck with big suction pumps and just let the structure sink into the hole.
Drive cooling pipes into the ground (mud) at either end of the preassembled portion and freeze the muck. Then excavate it using the usual shield and lock techniques.
The hardest part would be convincing the preassembled tunnel to submerge - and stay submerged once it’s full of air. IIRC, the tunnel sections of the Oresund crossing close to the Danish shore were ballasted with iron ore. For someone into maritime modeling, any part of the construction phase could be interesting to model. As for me, I’ll settle for hard rock tunneling in the mountains…
Chuck (Native Noo Yawka modeling Central Japan in September, 1964)
