Why would a railroad limit the tonnage behind engines and helpers on grades? Trains are supposed to have enough dynamic and air brakes to control a train going downgrade, plus with the helpers attached that would prevent bunching of the cars behind lead locomotives.
EXAMPLE: In the SP Northern Region Timetable for the Cascade Subdivision
Territory Road Engine Helper Engine
Chiloquin-Kirk (E) 10000 8500
Dougren-Minnow (W) 10000 8500
Oakridge-Cascade Summit (W) 4500 4500
Tonnage rating are just that: how much a given horsepowered locomotive will haul on a given portion of railroad based on the overall gradient. Specific increases in gradient will call for more horsepower. In the examples above, the addition of a helper engine merely maintains the tonnage status quo. Up grade it is the horsepower needed to lift, downgrade it is the braking power to keep the speed in check and the train undercontrol.
Two things come immediately to mind. First; the strength of the couplers. Too much tonnage and a knuckle breaks or a drawbar pulls out. Second; the possibility of a stringline derailment on the curves. (Not to mention the added resistance of pulling a long train around multiple curves.)
I’m sure someone else will deliver a more detailed explanation.
Also; this former SP line has timetable directions that do not correspond to the actual compass direction the train is going. (The line is a north-south line from Sacramento to Portland OR - I believe the restrictions shown are for the upgrade; not downgrade.)
Upgrade reasons:
Downgrade reasons:
Tons per operative brake
L/R limits
Air-brake recharge time
Air-brake recharge time in cold weather
Train line leakage rate
Train dynamics issues
RWM
1a. Most vulnerable just after the units crest the hill and begin descending, unless the engineer throttles back as soon as he tops the hill (the gravity pulling on the part of the train that has crested the hill adds to the pulling power of the locomotives, greatly increasing the stress on draft gear components).
Yup. We had a rash of break-in-twos on hogbacks right behind the head-end when we went to ACs. Then when we went to DPUs, another rash. We certainly proved how good ACs were at ripping trains in two!
RWM
Signifigant changes in motive power characteristics mandate different train handling procedures over difficult territory…both upgrade and downgrade. Unfortunately, these different procedures are learned by following the old procedures until they don’t work.
Good Train handling comes from experience…Experience comes from failed train handling.
Chiloquin - Kirk, up hill, max grade 0.8% 10 miles for then down hill max grade 0.2% for 3 miles.
Dougren - Minnow, up hill, 9.5 mils max grade 0.8%
Oakridge - Cascade Summit, up hill, max grade 1.8% for 44.5 miles
These figures are based on the grade drawings in he timetable.
See also Al Krug’s “Railroad Facts and Figures” webpage, “How Much Force can a Coupler Withstand ?” at:
http://www.alkrug.vcn.com/rrfacts/drawbar.htm
His “Second” reason at the bottom of the page echoes what zardoz and RWM said above about coming over the top of a grade.
Also; this former SP line has timetable directions that do not correspond to the actual compass direction the train is going. (The line is a north-south line from Sacramento to Portland OR - I believe the restrictions shown are for the upgrade; not downgrade.)
Yes. To the SP, you went east when you were going away from San Francisco, and west when you were going towards San Francisco–the actual compass direction did not matter. If you look at a very old Amtrak timetable for the Coast Starlight, you will see that the LA-Seattle train was numbered 13-14, and the Seattle train was numbered 11-12; the direction changed at Oakland. I have not examined closely a timetable for central California, but I am sure that you would find interesting timetable directions as opposed to actual compass directions.
Also, I know of at least one instance in which the UP has changed the mileposts–between Sacramento and Stockton.
Johnny
I am very familiar with the SP directions, having lived near San Franisco for years and ran in the locomotives of various types as far as Watsonville to Oalkand to Roseville. I now fully understand why tonnage was limited over certain grades. Can anyone tell me if I am correct that you add 0.4% to make it a compensated grade? I beleive that is the figure. I do not believe that the curves over Donner Pass, which I know intimately were superelevated, which would help prevent stringlining.
The usual compensation formula is 0.04% per degree of curve sharpness.
Passenger speed limit on Donner is still 30 or more, isn’t it? That wouldn’t be legal on 10-degree curves with no superelevation.
See also Al Krug’s “Railroad Facts and Figures” explanation of “Dynamic Braking Force vs Locomotive Speed”, at the bottom of his webpage on “Amps vs Tractive Effort”, at:
ConRail’s Industrial Sidetrack specs required grade compensation of between 0.04 and 0.05 % per degree of curvature - I always used the latter figure. Thus, a nominal 1.50 % maximum allowed grade on tangents would - in a maximum allowed 12-Degree, 30-Minute curve, be reduced by 12.5 x 0.05 = 0.625 % - so that the actual grade would be 0.875 % (1.500 - 0.625).
Conversely, if a curved track is built on a grade without such an easing of the actual grade as compensation for the curvature, then - for going upgrade - the appropriate compensation figure should be added to the actual grade % to obtain its equivalent grade % for train resistance calculation purposes.
Going downgrade, the compensation figure would be subtracted from the actual grade to assess its effect on train resistance. Thus, in my example above, going downgrade through that same 12-Degree, 30-Minute curve on an actual 0.875 % grade would feel to that rolling stock in the curve as being a 0.250 % downgrade (0.875 - 0.625).
All of this is just an attempt to make allowances for the real-world physics effects - it reflects practical experience, compromises, and adjustments, and is by no means perfect or completel consistent. Here, reducing the uphill grade to compensate for curvature so that an ascending train is always '“feeling” a consistent grade through both tangents and curves is thought to be preferable to doing that for a downgrade train - thus a downgrade train encounters much different braking requirements as it enters and leaves each curve and tangent.
A 10-Degree mainline curve without at least some superelevation is pretty much unheard of. Without consulting a chart, I’d estimate that about 1-1/2 inches would be required for 30 MPH. But on a steep grade, the “optical illusion” created by the track curving as it rises or falls may make