What forces does a freight train experience?

Question

Can anyone tell me what sort of forces a load on a freight train experiences during travel and marshalling? I'm thinking of the magnitude of draft and buff forces, what sort of G loads need to be allowed for (for instance in specifying tie-down systems)

Answer 1

Excellent question.

The primary forces act in all three dimensions, but the biggie is a ratio between vertical (downward) force, ie the weight of the car, and lateral force, such as when traversing a curve.  The formula is simply expressed as L over V, or L/V.

As long as the vertical force is greater than the lateral force, the equipment will stay on top of the rail.  But, if excessive forces develop in draft, then the cars will want to pull off the inside of a curve, called "string-lining."  For an easy demonstration, get a couple pieces of model train track, and lay a length of string between the rails.  When you pull on it, it will stay inside the rails.  But, pull it then put a finger on the other end as you do so.  The string will want to straighten into a straight line, no longer between the rails.

When in buff, the same ratio applies, but here excessive forces will cause "jack-knifing," where the equipment will want to climb the outside of the rail.

Either way, if L/V is exceeded, a derailment is highly likely.

The third force is, of course, gravity.  Here too there is a simple formula to determine power requirements for a specific amount of tonnage up a given grade.  It is called the "Rolling Train Resistence Formula," and is as follows:

Horse power per ton, times 12, divided by the per cent of grade will equal speed, or HPT x 12 / %G = S.  

As speed decreases, forces increase, exponentially.  So, if ascending a grade with a given amount of tonnage, and we want to keep our speed high enough to avoid increasing forces (ten MPH is when forces start to get higher than wanted as well as resulting in locomotives working in 'short time', where there is danger of burning up traction motors), we know how much power we need to put on the train to get it up the hill within defined force limits where speed is a factor.

As far as securing lading is concerned, slack action creates the highest spikes in the forces at play.  Most open cars, such as those carrying lumber, have several tie down chains that secures the load, as well as being equipped with bulkheads to further restrict the load's movement.  But, shifted loads occur all the time, and present the greatest danger to those hapless individuals seeking cheap transportation.

Next time you're waiting for a train to pass, look at the ends of the equipment.  When you see wooden bulkheads that have been blown to bits, or the end of a box car all pooched out, these have had a run in with excessive forces of one sort or another, usually during heavy-handed switching.

Other commodities, such as automobiles, for example, also have tie down chains securing them.  Some box cars have locking dividers inside, while others have nail-able steel floors so blocking material can be secured.

Answer 2

haha, this is actually a very good question but as a lifelong railroader I can tell you there are some pretty high forces, not every day but I have observed some very strange things.

I really think you should contact the specific RR you are wanting to ship with, they can give you instructions, after all, they dont want anything falling off their cars.

Answer 3

Wind resistance and friction are 2 that it would experience.