unit trains

Before I answer your question, I must clarify something.  When you mention the term unit trains it is kind of an umbrella term that refers to a single train that can carry any commodity besides coal.  There are unit trains of grain, ethanol, of course coal, and others.

Now, to answer your question when a coal unit train is dumped, the process is somewhat automated.  The cars are parked along a positioner that moves the cars to be dumped.

watch?v=jdStoBp6Sw

In regards to whether an entire train is broken up, as far as I know they are broken up, but someone may know of another instance where they unloaded without being uncoupled.

If you look at a typical unit coal train, note that one end of each wheeled bathtub is painted a contrasting color.  The coupler on that end is a rotary coupler, and the cars can be rotated without being uncoupled as long as there is a rotary coupler at the end of the rotary dumper barrel.  The coupler can be on the car in the barrel or the car standing on fixed ground - but one or the other has to have a rotary coupler.

If you use your favorite satellite viewing system to look at the loadouts at modern mines you will notice that they are usually located on reverse loops.  Likewise, the destination facility will have its receiving track in the form of a loop.  The entire train turns 180 degrees in the process of being converted from a loaded unit to an empty unit.

Chuck (Modeling Central Japan in September, 1964)

I have seen unit trains cut into blocks at older loadouts and during the unloading process where a loop wasn't feasible or cost prohibitive.

A unit train is different from a train that happens to carry all the same commodity. As much as possible, unit train means the train is kept together as a unit - coal is picked up by the train at point A and taken to point B to be unloaded, then returned to point A for more coal. A 1950's iron ore train going from the Mesabi Range to Duluth, Two Harbors, or Superior isn't really a unit train, because it's made up of cars from several different mines, and the cars will be broken down into different cuts to be unloaded - and some may even be interchanged with another railroad. It might look like a unit train, since it's 100+ cars of the same railroad carrying the same thing, but it's technically not a unit train.

tomikawaTT

Likewise, the destination facility will have its receiving track in the form of a loop.  The entire train turns 180 degrees in the process of being converted from a loaded unit to an empty unit.

Not true in every case -- Tucson Electric's tracks are dead-ended sidings.  They have their own switcher to maneuver loaded cars from an inbound loaded track, through the rotary dumper, and then shove the empties onto the outbound track for later pickup.  They break the inbound train into sections because their dumper's track isn't long enough for the entire train.

In Jeff Wilson's book 'Industries Along The Tracks' che has a section on coal customers, which includes coal-fired power plants. In the article is a series of photos of the Tecumseh Energy Center (Topeka, Kan.), two photo's of particular interest are of a covered hopper unloading spot, the second photo being the plants siding off the main with coal hoppers on and the caption mentioning unit trains of coal are left on the siding and brought to the unloading area in the plant 5 at a time by a 44-tonner industrial switcher.

Trains Magazine a while back had a special on the coal industry. It was a good issue and talked about how unit coal trains were unloaded. I can't comment more than that because it's been a while since I've read the article and can't remember where I put it.

wjstix

A unit train is different from a train that happens to carry all the same commodity. As much as possible, unit train means the train is kept together as a unit - coal is picked up by the train at point A and taken to point B to be unloaded, then returned to point A for more coal.

Not contradicting Stix on his description, a unit train technically is a train which is billed on a single waybill.   So multiple cars are billed from the same origin to the same destination on the same single waybill.  Since it is one shipment of one commodity from one origin to one destination on one waybill it tends to move together. 

"units" can be as few as 10 cars, but mostly they have been in increments of 25 cars, up to 110, 115, 125, 150 cars.

A 1950's coal train will individual cars billed on individual waybills.  So even if they had 100 cars of coal from the same origin to the same destination it wouldn't be a unit train because it wasn't on the same waybill it would be on 100 different waybills.

Also remember  unit trains gets a special shipping rate..

BRAKIE

Also remember  unit trains gets a special shipping rate..

....and that's why they didn't show up until they did, there was no reason for them until the ICC allowed a discount rate for the unit.

3155944

I understand that unit trains are coupled all the time. Also I have read that they go through a rotary unloader to unload.Do they stop and start for each car or is it a continuos slow roll through. which ever method is used it must be time consuming to place each car just so.

There are 5 common methods for unloading unit coal trains:

1. Rotary dump -- either single car or tandem (two car).  Both use an indexer to advance the train 1 or 2 cars at a time into position.  The train is stopped while the car dumps.  While many receivers equipped with rotary dumpers can accept an entire train without breaking the train apart for dumping, a substantial number must break the train into pieces for dumping.  If you see a switch engine at the receiver, it's a pretty good indication it's breaking the train apart and needs the switch engine to move cuts of cars through the dumper.  Rotary couplers enable one car to turn over while coupled to stationary cars to either side.  Typically a single-car rotary dump has a maximum throughput of 4,000 tons per hour, and a tandem dump 6,000 tph.  Rack-and-pinion indexers are very accurate regardless of temperature and there's usually not much messing around with stopping the train with the car to be dumped in the right position.  Cable indexers are not as accurate and there might be some back-and-forth to get the car into the correct location especially at the beginning or end of the train, or if there's any gradient in the dumper track that the train wants to roll down, or be pulled up.
2. Rapid-discharge.  These are bottom-dump cars that are pulled through the dumper typically by the train's road power.  The hopper doors are electric-over-air controlled.  A shoe on the side of the car contacts a rail to the side of the track (and about 5' above ground level) that is electrified.  Current flows from the rail through the shoe and opens an air valve.  Compressed air enters cylinders and pushes pistons that open all doors on the car at once.  The car empties in about 2 minutes. The train can move continuously but often it stops while the conveyor system catches up.  Like rotary dump receivers, many rapid-discharge receivers can accept an entire train for dumping without breaking, but a substantial number break the train apart.  Rapid-discharge can unload a train very quickly IF the dumper has enough conveyor capacity, or coal-moving machinery (like D-11 carrydozers) to move the coal away from the dumping area as quickly as it's dumped.  Typically the limit is the conveyor system, and typically these are not better than 4,000 to 6,000 tph.
3. Rotary single-car. These are commonly seen at ports and steel mills and in many cases are of relatively old construction.  Cars are typically individually pushed into the dumper by an "electric mule", a small electric locomotive running on its own (typically narrow-gauge) track next to the dumper track, then exit the dumper by gravity and roll into a receiving yard.  Cars dumped in this kind of a dumper can be almost any type including gons, so long as they will fit into the dumper and the dumper arms can grab the top of the car without damaging it.  The cars don't need to have rotary couplers (but you'll often see rotary-coupler cars here anyway).  Capacity varies wildly.  Some of them are not much better than 1,000 tph, others can do 6,000 tph. 
4. Standard bottom-dump.  These are hoppers with manual doors that are opened and closed by hand.  Most receivers equipped only for cars with manual doors break the train into cuts and shove them over the pit, but I've been to a few that don't uncouple the cars.  Trains must be stopped while each car is dumped.
5. Top excavation.  This consists of digging the coal out of the cars with a hydraulic excavator or a clamshell bucket on a crane.  This doesn't work for hoppers because they have internal crossbraces, but it's just fine for lowside gons and high-side gons that don't have internal crossbraces.  This is a very common method for rock and aggregates moving in unit trains but by no means unknown for coal.  Sometimes the excavator travels down the top of the cars, and sometimes the train is moved past the excavator or clamshell crane.  Trains must be stopped while the cars are dug out.

Unit grain cars are gravity bottom discharge covered hoppers.  The train is stopped with one or more cars over the dumper pit, the doors manually opened, then closed after the car is emptied.   A surprisingly large number of high-throughput export elevators must break trains apart for unloading.

Unit soda ash, potash, and salt -- all dry, granular, bulk commodities -- typically move in cars similar to those used for grain, and typically dump similar to grain.

Sulfur when moved in solid form in unit trains usually moves in rotary-dump, high-side flat bottom gons just like coal.  Same goes for sewage sludge.  Both are typically rotary-dumped.

Taconite has moved long distance in unit trains in ordinary steel hoppers that are rotary dumped or bottom dumped.

Liquid commodities moved in unit trains -- crude oil, sulfur, ethanol, asphalt, brine, etc. -- is typically loaded and unloaded in multiple car blocks at racks.

RWM