Freight Train Questions

Or helium-filled zeppelins.

I don’t believe there are any legal limitations to the length of a train. Union Pacific ran a 3 1/2 mile long train from Chicago to LA to see how it would work out.

Here is an article from the LA Times that puts it bluntly: "There are no state or federal limits on the length of trains or requirements to notify agencies about unusually long train configurations, officials said. Union Pacific said it did alert local federal regulators, who observed the train’s movement. "

Keep in mind that railroads are (generally speaking) privately owned and (generally speaking) do not run on publicly owned tracks.

Very interesting thread, always learn something new on this site.

Something that has been puzzling me and though it’s a bit of a hijack as there are some railroad experts here perhaps I can post the question.

I may be a dumb question but with the weights of freight and length of trains involved how do they ever get the whole train moving?

The engines have steel wheels riding on steel rail, I don’t know the number of driving wheels an engine may have but it can’t be more than say 10, the actual wheel to rail contact can’t be more than a few square inches.

How can this be enough grip area to get a two mile long train rolling?

What am I missing?

Sand, electric motors, and the weight of the engines.

Gotta love the SDMB. I was just at a railway museum and wondering what the tubes that come down in front of the drive wheels on many of the engines where for. I assumed it had to do with traction but but thought it was perhaps something related specifically to steam engines.

Turns out it is for delivery of the sand ironbender mentions. Love when my ignorance is fought.

Mauritania is supposed to have the world’s longest and heaviest train. I don’t know if it’s a true claim but here is a link

Slack coupling makes the laws of inertia work for you rather than against you. The locomotive gets the first car moving and once it’s underway a fraction of an inch, the coupler pulls taut and starts the second car moving, and so on down the line. Anytime you’re next to a long freight train that’s starting up you can hear, moving from front to back of the train, the sound of the couplers tightening. It will take a couple of seconds for a long train. This means that the very last car gets quite a jerk, going from a stop to 15 mph in an instant, and was one of the reasons cabooses were dangerous places to ride. Generally the conductor and brakeman would hear the couplers coming toward them and have time to brace for the wild start.

For this same reason, slack coupling is not used on passenger trains.

How is that avoided?

Passenger trains use close coupling, with no slack in the knuckle. For one thing, they have bellows fittings around the train doors, allowing passengers to walk from car to car. I’m not sure exactly what hardware is used on the couplers to ensure things are tight. Really modern trainsets, as used on high-speed trains in Europe and Asia, are articulated trainsets where a set of 9 or 10 cars are permanently joined together, with the trucks (sets of wheels) under the “coupling” rather than at both ends of the car. This reduces friction and gives a smoother ride. Lengthy high speed trains will have two trainsets coupled together. Each trainset has a “locomotive” at both ends, so passengers from the front trainset cannot reach the rear trainset while rolling.

Here’s a coal train over 7km long. According to the description it has 8 engines and a special control system to keep them in sync, but only one driver.

You’d have to be a really big fan of trains to watch the whole video.

OK sand and weight of engines I can understand but what does the electric motor do? and what about the great age of steam before they had electric motors?

The slack couplings make sense. I remember as a kid hearing that clonking down the line of a freight train as it moved about, thought it was just loose couplings didn’t realize it was part of the system.


Modern locomotives are diesel electric - the diesel engines turn generators to provide electricity for the electric motors which drive the wheels. One reason this is done is that an electric motor can provide full torque at low RPM, whereas an ICE is only effective over a narrow RPM range. Steam engines operate well at low RPM also.

They had steam engines, coupled to the wheels.

In modern diesel-electrics, the diesel generates electricity from fuel (oil) and the electricity powers the driving wheels. I’m sure someone will tell us why, but I can think of at least one reason. Electric motors operate efficiently over a wide range of speeds, so don’t require a transmission. Reciprocating, internal combustion engines run well only at a narrow range of speeds, and need a transmission to cover a wide, more practical range.

The transmission was particularly a problem in the 1930s as diesel-electrics were being developed. Small industrial locomotives sometimes do use transmissions and directly driven wheels, but a mechanical transmission for something with the power of a 3800-horsepower locomotive is just not practical. Switching equipment to control electric motors was already well-developed, though. A related problem is powering multiple sets of driving wheels.

As to why, you are correct that an internal combustion or diesel engine develops torque as RPMs increase (to a point), thereby creating the need for a transmission. Electric motors develop torque instantly. Perhaps some more knowledgeable than I can tell me if or to what degree torque increases with RPM levels with an electric motor.

Anyone know what RPM ranges the diesel engines and electric motors in locomotives operate within?

They would need a heap of power.

I saw a show on tv that said steel wheels will start spinning on the rail once traction is broken. It went on to say that an advantage of the diesel generator/ electric motor is that traction and torque can be monitored electronically to provide maximum torque and can be instantly adjusted if the wheels start to spin. Don’t know if it’s true or not.

The show also mentioned that they carry sand in tanks and can blow it under the wheels for extra traction, as others here have stated.

Can I put out just one more question while we’re all here?

The old steam engine always seemed to give their wheels a quick spin before starting to move, what was the reason for this? Laying sand to get grip before moving off perhaps?

Can’t somehow think it was “laying down rubber” like a drag racer :smiley:

It look good in the movies. A good engineer would never spin his wheels, to expensive.

(Former steam locomotive engineer at your service)

Wheelslip on a steam locomotive is undesirable, mainly because you can generate enough heat via friction to actually throw off the driving wheels’ tires. Yes, tires.

In looking at a driving wheel, you’re actually looking at a couple of pieces. The hub (and spokes, if any) and the flanged outer ring (the tire.)

The pieces aren’t joined by welds, bolts, screws, or anything else. They’re sweated on. Large rings of fire heat the tires to expansion, and then they’re placed over the hubs. As they cool, they contract until eventually they are tightly attached to the hub.

Spinning the wheel builds up heat, which expands the tire…and you can spin it right off.

As to why steam locomotives tended to spin the drivers, it really boils down to the guy on the throttle. Since it takes a lot more power to start the train than it does to keep it moving, you’ll be opening the throttle quite a bit if starting a heavy train (moreso on an incline. Moreso if the slack is stretched.) The difference between “enough steam to get us moving” and "too much steam and spinning the wheels) is, really, a fraction of an inch of throttle travel. When they start to spin, lots of times the only way to stop them is to completely close the throttle and start again from scratch.

What’s even crazier than that is to watch some videos of articulated steam locomotives, which have 2 sets of drivers (essentially, 2 engines) under one very long boiler. Union Pacific’s “Big Boy” 4-8-8-4 or “Challenger” 4-6-6-4 locomotives are good examples. Lots of times, one set of wheels would spin while the others wouldn’t. Looks mighty funny.