New to forum, but have been interested in trains for the last 20 yrs, or so. I am now on my 3rd layout, after finally doing research, rather than just digging in with no definitive plans whatsoever. My question is simple, but may attract many different opinions. I’m working on a PRR based city layout. It’s a freelance using the town/city that I grew up in, as well as the locale that I live now. I’ve planned on several bridges, including both train and auto overpasses. What is the general concensus in constructing said bridges; does one lay track first, then build the bridges “under” the existing rails, or should I cut away the pre-laid (and tested) track, and install bridges later? I’ve been fighting this problem for weeks, if not months. Lately, I’ve just been observing my just-started layout for hours at a time, and can’t figure out which approach may be the best for me.
I lay my track first but I’ve seen it done both ways. Try each way and see which you like. The worst that could happen is that you’ll have two bridges built. Good luck.
Since I ‘imagineer’ my scenery first, I provide for bridges during initial construction. My prototype ran to deck girders, typically painted oxide red, so it’s a fairly easy matter to lay track with bridge ties over the black-painted solid cores which will later be detailed with a thin overlay supporting the angle irons, flanges and rivet detail needed to accurately portray the girders. The typical piers were masonry - again, an overlay covering the pre-positioned vertical supports. Steel tower piers can be installed later - even years later. (John Armstrong operated his trains over his ‘bentless trestle’ for a long time.)
Truss bridges, with their open floors, require a bit of additional pre-planning, in the form of a recess under the bridge site so the rails will go down level. The trick is to provide quick-removal joints in the temporary roadbed/subgrade where the bridge and its abutments will go. When scratchbuilding, I lay track over the pre-positioned floor assembly, then add the above or below-rail truss structure later. If possible, the rails across the bridge should be continuous (not practical for long trestles or viaducts, but not difficult for the shorter bridges I’m modeling.)
Adding a bridge where one hadn’t been provided for in initial construction is a bit more complex. One useful trick is to model a ballasted-deck bridge - just saw the subgrade and roadbed along the lines of the edge girders, cut away the rest of the immediately-adjacent terrain and rebuild with bridge structural members and appropriate below-the-rail terrain. This technique will allow construction of anything from a highway overpass to a high concrete arch below the solid deck.
Just a few suggestions, not meant to serve as detailed directions for construction.
Chuck (Modeling Central Japan in September, 1964 - with LOTS of bridges)
I am currently building a PRR Prototype Stone viaduct bridge made out of hydrocal from Model Railroad Stone works. Below are a few construction pic hopefully it will be finished this weekend. My personal method of madness is to build and place all my bridges first and have the bench work meet them.
What I generally will do on sections that have track already in place and I’ve changed the plan to put in a bridge is cut out the required section of the subroadbed etc. just a bit larger then the span of the bridge.
Every other bridge on the pike has been build, detailed, weathered and had track laid on the work bench but this latest bridge is far too large (38" long, and way too heavy (20+lbs) so it has to be built in place. I removed a section of the bench work, so I could work in close to where the bridge is going to live. It’s way too much to be leaning over completed bench and track work etc. a very good example of why good planning pays good dividends. I don’t know if it’s apparent in the photo’s or not but the bench work has been reinforced to the point where I can park a small truck on it without so much as a creek coming from it. One of the hazards of working with hydrocal is it does not flex at all so the least little bit of flex and you have a pile of white dust that will need to be cleaned up…
So for my money bridges are a very intricate part of your track plan and it is in a lot of cases much more convenient to build up to the bridge rather then remove track and bench work etc. to make the bridge fit. I think this applies to just about all types and styles of bridges not just big heavy monsters like what I’m building and this isn’t even the biggest one planned for the pike.
Good luck on your new railroad and make sure you have lots of fun building it.
Plan ahead with flexibility – It is like licking your finger, sticking it up in the air, to see how the wind blows. So, if you are buying “retail bridges” – know the dimensions of the bridge to be purchased, and work accordingly from there for bridge pier placement. Again, just be flexible for each bridge type & location.
Example #1: CR&T’S community of Conemaugh, will use the (plywood) cookie-cutter technique for 2 bridge overpasses…
Sub-roadbed “benchwork pier supports” will be securely attached before cutting out the top surface for a Warren Truss Bridge, and; just a few inches later, where a Plate/Girder Deck Bridge will be cut out.
Example #2: Dave Frary in The Pennsy Middle Division in HO Scale illustrates how he kept the overpass/bridge sub-roadbed in-place while attaching bridge deck plates to its sides. This is a highly suggested out-of-print purchase on eBay if you are modeling the Pennsylvania.
Pennsy Prototype: There is a difference between spanning a small river, as in Dave Frary’s book, and the construction of a PRR stone bridge, like Allegheny 2-6-6-6’s really neat viaduct – Note his and tomikawaTT’s rock-solid rationale.
See this circa 1960 picture of the PRR’s Stone Bridge in Johnstown, PA, with a more elevated Pennsy 4-track maineline approach – common between Altoona and Pittsburgh, and; Greensburg’s mainline viaduct is even higher than Johnstown’s Stone Bridge.
I use plywood for the subroadbed. Where there is to be a bridge, that portion of the subroadbed is cut out for the bridge to take its place (unless the subroadbed an integral part of the bridge.). The cut-out roadbed can be helpful in constructing the bridge to fit with the abutting track.
If bridge building is delayed after laying the track, the track, sans subroadbed, is preserved if (1) the bridge doesn’t have a “top” like a thru-truss bridge and (2) the bridge has a ballasted deck using normal ties. Otherwise, the bridge’s rail and ties will need to be installed as an integral part of bridge construction.
Ditto, except in my case I removed a section of spline roadbed. Same diff, same rationale, same iterative fitting process for a timber trestle I wanted to insert. Where I cut the spline roadbed, I built a timber abutment at each end, established and looking like it was erected in extant scenery which I had roughed in. All I had to do from that point was lay the trestle ends atop each abutment, or try to, and if it was still high, snipped off small bits of the bent frame at the lower end until I met the grade up top.
The way I did it was to lay the track and then later install the bridge. Once I got the track work the way I wanted it, I then put the bridge in place.