Why is the new bridge a closed deck?... i.e. it appears to have a solid concrete deck with the track and ballast placed on top ...verses the old bridge which was an open deck.
Also, for some reason the new bridge doesn't (yet) have the center rails to prevent cars from falling off in the event of a derailment.
(Ballast Deck is preferable to an open deck structure. Surfacing is much simpler and cheaper. The transitions at the ends are easier to maintain...Open Deck structures ought to be phased-out or banned. If the bridge can handle the extra dead weight - ballast deck the darned thing.)
Guard rails may have been removed until final surfacing or the replacement secondhand rail either had not arrived or been installed by the local track forces. G/R should be of similar height or slightly taller than the running rail and extend a set distance off the bridge. G/R placement is a call the railroad chief engineer sets a standard for, not the FRA.
Installing and maintaining/ replacing the special bridge ties for an open-deck bridge is an expensive headache. Thay all have to be 'dapped' (notched), often only a few of a kind if the stringers have varying thicknesses (not likely for an arch bridge, I'll concede). Then there's the spacer blocks between them, the tie bar above, the hook bolts to hold them to the stringers or main beams, the walkway grating, the tie pads between the tie plates and the timbers, and the drilling and hardware for all of the above. Plus, if guardrail is installed, that and the guard timber on the outside.
It may be a judgement call by the ChEng's staff that no guardrail is needed. It's a modern structure with excellent welded rail track (and frequent defect testing) on a tangent, moderate speeds (35 MPH?), and freight only. Plus, the 'curbs' of the precast ballast deck will serve as a guardrail of sorts.
- PDN.
In my area most (all?) open decks have been and still are constructed of creosoted timbers, which of course present a fire hazard.
Pretty hard to get concrete and ballast to burn!
Greetings from Alberta
-an Articulate Malcontent
Good answers.. thanks folks.
I believe the original purpose of guardrails was to prevent a derailed train from striking the bridge superstructure, which could turn a simple derailment into a bridge collapse. Since none of the arch bridge structure is above therail surface, then perhaps they were deemed unnecessary.
Ballast decks seem to be the standard now. South Shore's new bridge in Hegewisch over Torrence Ave. and the NS (NKP) is ballast decked. CTA's Midway L (Orange Line) is ballast decked on all of the new bridges on that line.
adkrr64 I believe the original purpose of guardrails was to prevent a derailed train from striking the bridge superstructure, which could turn a simple derailment into a bridge collapse. Since none of the arch bridge structure is above therail surface, then perhaps they were deemed unnecessary.
Ballast deck bridges are sooooo much easier to maintain and adjust tracks on. The G/R's keep the wheels from wandering off the ties and off the bridge. You pray it's a light railcar that derails at any kind of speed.
(The old Santa Fe Chief Engineer's instruction was any bridge where the B/R was 16 feet above the bottom of the opening or within 12 feet of a main track centerline and a critical structure. Not sure what BNSF uses these days.)
Perhaps my memory is faulty, but I recall reading many years back that the guard rails are there to reduce the distance that derailed cars move away from the track--and on bridges, especially, you really do not want such cars to go very far from the rails (I am confident that most readers of this are aware of this).
Johnny
On the Washington DC Metro's partially elevated stations the parts of the track that are on the elevated portion have check rails on them.
The same issue of Trains also shows a photo of Starucca Viaduct with out guardrails.
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