Is the track designed to move up and down as a train goes over? What would be the reason for that? I’ve noticed that on some stretches the track is perfectly rigid while on others there’s quite a bit of flexing.
Properly installed and maintained track should not move up and down as the train passes over it.
Used to be a section of track just past a grade crossing where I railfanned a lot that would ‘pump’ like that. In wet spells, water would squirt up 6 to 12 inches into the air from around the ties and in dry spells white dust would puff out and drift along with the moving train.
I often saw a tamper stop at that place to re-tamp the ballast, but it wasn’t long before the pumping started again. I think it was a fill area that used to be a minor creek along side the road and that a lot of limestone rock had been used to fill it in, with the assumption that they could get the natural flow of water to follow a route to some tiles under the track a short distance away, but Ma Nature doesn’t pay much attention to man’s meddling.
I use to see that a lot while riding a bike on a rail trail right next to the Pan Am Railway main line in Northampton, Ma. Many, many spikes were at least a half inch above the rail flanges. The max speed was 20 mph. Freight use only for many years. Scrap iron and coal trains. They just stopped coal trains. Last coal fired power plant now cold. Some general freight and oil tank cars.
Now Amtrak has been rerouted over this area with new 136 lb ribbon rail and Pandrol clips, ballast. No more spikes.
Rich
Depends on the class of track…class 3 and up should have very, very little to no pumping…as for yard tracks, and some short line/regionals, as long as the track stays in gauge and the movement is not drastic, no big deal, it will be on someone’s fix list.
Track modulus at work.
Track is a dynamic structure and does have some give/flex to it. It isn’t just a big flat chunk of concrete. The loading and reactions are different (and there aren’t pneumatic rubber tires to lessen impact loadings.)
The issue is when the “give” is excessive or less than uniform.
“+1”.
To expand a little bit: The track really should have some flex - that’s how it spreads the load from the ‘point’ source of a wheel to the 2 - 4 ties under and adjoining it. If it didn’t flex at all, that load would be concentrated on 1 or 2 ties at a time right under the wheel, and perhaps overstress them. The complete answer requires a knowledge of the loads and stresses of indeterminate structures on supports of varying stiffness, analyzed by differential equations or numerical methods - probably at least a senior level course in a 4-year college, or a master’s degree.
As MC implied, it’s a bit of a Goldilocks “balancing act” - not too much, not too little, but ‘just right’ (hopefully).
- Paul North.
As an aside - ever sat on a bridge, with traffic moving on the other land(s) - a whole lot of shaking going on. When you are driving over the bridge - it still has all the vibrations & shaking, you don’t feel it.
Transient loads have to be handled, no matter the structure, when the loads are great enough the can be seen and/or felt.
And that is a spooky feeling - standing on that big, solid, concrete and steel structure and feeling it move…
It’s the same in concrete parking garages with multiple stories, especially the top outdoors floor, say at a shopping mall. Felt like an earthquake if a 5 ton delivery truck happened to move two or three rows down.
Most building materials are elastic, and are usually loaded to near or at their capacity for economic reasons (“Don’t use any more than is necessary”). So flexing that can be seen or felt (“deflection”, vibration, and “bounce”, etc.) is inevitable, with the possible exception of large, short, stiff, massive masonry or concrete structures.
But most structures - and rails, as in this instance - are similar to a suspension bridge: no matter how hard you pull on the rope or support it at the ends, it’s always going to have some amount of sag in the middle.
The key questions - as always - are: “How much are you willing to pay for this structure with a reaonable degree of safety ?”, and then: “How much additional are you willing to pay to eliminate all perceptible flexing ?” The answers are usually “As little as possible”, and “Zero”, respectively.
- Paul North.
On the Canadian National’s Grand Trunk Western line North of Vicksburg, Michigan there is a grade crossing on UV Avenue. Mainline 1 has a spot that always bounces. The spot is near a signal cabinet.
The sides of the rails are coated in a lighter colored dust than the rest of the tracks.
Mudhole after it dries out. Possible beginning of a ballast pocket situation fed by signal department disturbing the hardpan with their cabling to the signal case. (and accentuated by the change from stiff to soft to stiff modulus pumping…Mooks sees that at her “spot” in Nebrasky frequently)
OMG- Signal Department actually dug-in their cables to the prescribed depths?[swg]
A common specification for maximum floor and bridge deflection [EDIT] under “live loads”- the amount or distance that it actually moves downward - is L / 360, where L is the span length of the floor or bridge between supports. Sounds reasonable enough, until you realize that for even a 15 ft. span (180 inches), that allows a bounce of 1/2 inch down and back up again, likely at an uncomfortable frequency, too.
After experiencing this effect in some rather high-end houses that we visited and toured, this conservative engineer designed and constructed most of the floors in my house so that the deflection is less than L/ 1,000. They may vibrate a little bit when I jump up and down on them (all 250+ lbs. of me) - see my post above about how it’s impossible to completely eliminate all deflection and vibration - but they don’t bounce. Cost a little less too, to do it that way - but that’s another story.
- Paul North.
In viewing some shows about road construction, I have seen styrofoam blocks being used to ‘firm up’ the sub grade - I suspect this is being used where soil conditions are somewhat less than stable.
Are any railroad carriers using styrofoam blocking to firm up their roadbeds?
Balt: Union Pacific and Trax in Salt Lake City. (replaced some really ugly exspansive clay soils around an overpass and out onto a long curved fill)
From PD North
A common specification for maximum floor and bridge deflection - the amount or distance that it actually moves downward - is L / 360, where L is the span length of the floor or bridge between supports. Sounds reasonable enough, until you realize that for even a 15 ft. span (180 inches), that allows a bounce of 1/2 inch down and back up again, likely at an uncomfortable frequency, too.
So Paul look at this youtube and figure the deflection. World’s tallest bridge. Cable stayed and almost longest spans.
https://www.youtube.com/watch?v=HOAcRrfW-8s
Wonder if a RR bridge will ever be build like this one ??
The train is supposed to float in the ballast under the load. While subway trains are on concreat pads a freight load is much heaver and varies by 40 to 50 tons and should spread out as needed like a boat in water.
Nope - not in any meaningful sense.
The ‘dead load’ (weight) of unoccupied track (excluding the ballast) is from roughly 200 to 700 lbs. per foot (concrete ties). A train can add ‘live load’ from 7,000 to 14,000 lbs. per foot (under the closest trucks of 2 locomotives coupled together).
Recall Archimedes’ demonstration - a body sinks until it displaces it’s weight in the fluid. That amount of added weight would sink your boat and drive it way underwater (12 to 20 ft. +/- for water, about 40% of that = 5 to 8 ft. for rock ballast, if it behaved as a fluid - no “shear strength” - but it doesn’t.) The disparity between the loading for empty vs. loaded track is one reason that styrofoam blocks won’t work under mainline freight tracks (unless they’re very deep, and the subsoil is slow to react to added and removed loads, etc.)
Your subway analogy on concrete pads is much closer to reality . Note that in your example, the lighter load has the better support, and vice-versa. The only place where the track even remotely ‘floats’ (aside from pontoon bridge crossings) is across swamps and other very soft ground along major rivers - and there it is horrible maintenance headache.
- Paul North.
I can’t think of a time when I’ve ever watched a passing train and not seen the track displace downward under the weight of each truck. What surprises me is how fuel efficient trains can be compared to rubber-tired road vehicles, given the fact that there is so much flexing of the rail and roadbed. It reminds me of seeing a toy car roll across a made bed. I would expect this flexing to create an awful lot of rolling resistance, but I guess the rail is a very efficient spring that gives back nearly all of the energy required to displace it.
Can anyone comment on this? I know the interaction of wheel and rail is studied in great detail by professional railway engineers.