The railcars in a unit train all carry the same commodity, originate from the same location and travel to the same destination. High-volume bulk materials such as coal, coke, iron ore and aggregates are typically transported this way, as well as bulk liquids like ethanol, crude oil and condensate. All the railcars tend to be nearly identical, inside and out. Unit trains save time and money for a railroad, and eliminates any delays and confusion associated with assembling and disassembling the trains at railyards at both ends of the operation. This type of service enables railroads to more effectively compete with truck transportation and river barges.
A unit train stays together except when cars are removed for maintenance, and sometimes trains are turned around to equalize wheel wear. As the name implies, a unit train positioning system handles the same basic train time and time again. The forces required to position a coupled train are acceleration, deceleration, friction, slope and curve, and are dictated here by the train consist, cycle time and track profile. There are two specific types of unit train positioners -- entry end and exit end. The terms refer to the location of the positioner relative to a rotary railcar dumper for unloading, which are full train/entry end, and empty train/exit end, although not all unloading operations involve a dumper.
However, when paired with a dumper, the entry end positioner requires the locomotives to spot the first car, and will position all the remaining cars except the last one. The exit end positioner requires the locomotive to spot the first three cars and will position all the remaining cars. A typical position cycle is:
- Accelerate the train.
- Move the train forward at a predetermined velocity.
- Slow down and stop the train.
- Engage the chocks.
- While the car is being unloaded by the rotary dumper, the positioner will raise from the coupling line of the train and travel in reverse to the next car coupling and lower the arm to this line.
- The dumper will signal completion of the dumping cycle. The chocks will release, and the cycle is repeated.
In most instances, the locomotive will stay with the train. Each different facility may have extenuating circumstances that actually dictate operating procedures.
As manufactured by Heyl & Patterson, a train positioner is a series of mechanical, structural and electrical components that are integrated into a coordinated and highly sophisticated system. A carriage travels back and forth along a large concrete runway to move the train in the desired direction. The carriage can either be a large steel slab or machined weldment, and usually has three carriage wheels to allow travel to and from the dumper, and also four thrust rollers to translate the forces in positioning from the train line to the runway. A large pivoting arm on the center of the carriage attaches to railcar couplings to move the cars. The carriage must be of sufficient weight and balance so that its center of gravity stays within the wheel base as the arm swings from the "down" position for moving the train to the "up" position to clear the railcars at the end of the dump cycle.
With a wire rope positioner, a rope haul system is used to tow the carriage back and forth along the runway. The rope system incorporates sheaves on the carriage, a terminal sheave close to the dumper and a rope drum system away from the dumper. The reeving system pulls the carriage both in the forward and reverse direction, and the drum is scored for the proper diameter of rope. A motor and reducer are coupled to the drum to provide the torque necessary for all the train positioning forces.
With a rack and pinion drive positioner, the rope haulage system and the rope itself are eliminated, but it functions in much the same way. A circular gear, or pinion, engages teeth on a linear gear bar, or rack, and rotational motion applied to the pinion causes the rack to move relative to the pinion, translating the rotational motion of the pinion into linear motion. This train positioner features multiple composite drives, so redundancy is inherent in the design. If a motor should become incapacitated, the positioner remains operational.
Wheel chocks are another component commonly found in train positioning systems, and there are generally two types. A simple chock system engages a wheel set of a railcar truck to hold the train during unloading. This type usually uses pawls and hooks or grippers to grab and hold the car and train in position, and is located relatively close to where the positioner moves the train and may allow for only two or three inches of mis-spot. This chock's capacity is generally low, about 25,000 pounds.
A more complicated holding system usually has a pivoting forward pawl system that engages the front wheel of the truck around the centerline of the wheel. In conjunction with the forward pawl is a traveling rear pawl assembly that pivots to move the entire train as much as nine inches. This system was designed to compensate for positioner mis-spot due to the positioner's location relative to the chocks, coupler slack and other track induced items. This type is designed with a holding force of up to 75,000 pounds.
When necessary and dictated by track forces, both chocks combine to limit the train line forces on the rotary coupling components during a dumper rotation.
Train line forces above 75,000 pounds require a holding arm. This is similar to the arm located on the positioner carriage that pivots onto the train line. The holding arm confines the forces to the train line, which is designed for such higher forces. When a holding arm is required, the track forces are generally very aggressive and should be reviewed beforehand for alternatives.
Heyl & Patterson offers a complete line of train positioners that provide state-of-the-art operation and efficiency. Without locomotive assistance, these railcar movers can can be used in conjuction with rotary dumpers or for a variety of loading, unloading and short-range transportation applications.