Week 10: Design of the Rail Cross Section

Text (14 pages):

• Ch. 3 Track Structure (32-45)

Supp. Text:

• Ch. 17 subgrade geometric design (293-298)
• Ch. 20 drainage (366-392)
• Ch. 21 ballast (393-418)
• Ch. 22 cross ties (436-468)
• Ch. 25 fastenings (562-591)

Objective: To gain knowledge of cross section components, understand their relative contribution to cross section design, be able to apply the concepts by designing a functional cross section

• Click here to see two typical rail cross sections (Fig. 13-4, Wright and Ashford)
• The function of the rail structure is to transfer all loads without settling during all weather conditions
• the measure of support strength is the _____________________, or, the force required (in lbs. per inch) to depress the rail one inch. Typical values are 6000 - 12,000 lbs. per inch

• Rail Guage = Wheel Guage + 3/4 inch = 4' 8 1/2"
  
• Must be kept in tolerance
  
• Click here to read a report on Guage restraint (not from Text). A new "split axle" can reduce derailments. See figure 7 for distribution of widening. Look at the dimensions of wheels and rails - how much widening will cause derailment?
• Impacts required superelevation

• To examine forces acting during curves, see Figure 12-7 from Wright and Ashford
• To calculate balanced superelevation, use E = ________________________ (feet, mph) or E = 11.9V²/R (mm, kph) where E = Equilibrium elevation, V = speed, and R = radius of curve
• See Wright and Ashford, Table 12-6, which provides metric values for E, given speed and radius R
• Use a max E of 8 inches (200 mm) (desirable 6 to 7 inches (150 to 170 mm))
• to determine overturning speed, use V = 2.24 R (mph and feet; assumes center of gravity = 7' above outer rail)

• Purpose of ballast:

1. distribute loads (depth 6 - 30 inches); loads are reduced from 100 psi just under the tie to less than 50 psi at the top of the subgrade
2. reducing the load allows the subgrade to hold up and not settle (load less than endurance limit)
3. anchor ties ("interlocking" is required, so crushed stone is preferred to river gravel)
4. provide drainage (use open graded materials, 1.5 inch diameter.) - problem with too many fines will be ____________________________________
5. control weeds and dust

• Ballast quality standards:

1. Wear resistance
2. Cleanliness (can be improved by use of geofabrics)
3. Frost Resistance
4. Unit Weight (e.g., slags)
5. Ballast will usually last for ________ Million gross tons (25,000 trains at 10,000 tons each)

• Click to see Rail-Tie Interface, from Armstrong
• Click to see Photo of Rail-Tie Interface (Wright and Ashford)
• can be made of wood, steel or concrete
• wood ties should be treated with preservative and pre-drilled for spikes
• Crosstie functions: (NOTE: ties are 6" - 9" in cross section, 8' - 9' long, and are usually spaced at ________ centers)

1. spread loads
2. maintain guage
3. prevent longitudinal and lateral movement
4. deflect to cushion impact
5. making small vertical adjustments

• 39' long (some double length, now)
• jointed or continuous weld (advantages of welded include decreased maintenance, higher speeds, smoother ride)
• up to __________ of continuously welded rail can be moved on special "rail trains" that "bend easily around curves"
• 115 - 140 lbs/yd.
• welded track must be laid warm, so it will always be under tension (compression causes sun kinks - see safety lecture)

• turnouts have only 2 moving parts (switch points), but they can freeze up!
• the frog is that part of the turnout that allows the flange to pass over the crossing track
• higher frog numbers are used in ___________ turnouts and facilitate higher speed (up to 24, 50 mph+)
• Click here to visit the Q.C., Inc. Home page - Click the Unitrac link to see a catalog of rail cross sections which contains photos and descriptions
• Click here to visit the Atlantic Track & Turnout Co. where you can see Rail, Trackwork, Crane Rail Accessories, and Manufacturing photos, descriptions and a catalog of rail section materials. It also has a program that calculates frog numbers!

• for jointed rail, alrternate bolt heads inside and out to avoid joint failure if a car is derailed and cutting inside bolts
• steel tie plates as long as 18" are used to prevent damage of concentrated loads on ties
• rail anchors (spring clips) are used to reduce "creep" - _________________ movement of rail
• purpose of spikes is to prevent lateral movement, not keep the rail from lifting out

Plan:

Profile:

Cross Section: