KarTrak

A diagram of a complete KarTrak ACI barcode plate, as would be mounted on a rail car.

Examples of the various KarTrak labels used to form a barcode.

KarTrak ACI tags consisted of a plate with 13 horizontal labels put in a vertical arrangement that are also understood as data lines, which could have 13 different forms. These labels, or symbols, stand for the single digits 0-9, the number 10 as an extra feature for the checksum line, and the "START" and "STOP" labels that gave reference to the vertical line position of the tag.[1] Present day available depictions of the labels do often name the upper color first and then the lower color.

In practice people found that there was a significant number of cases where the label set was not done correctly or the label application had errors such as a 180° rotation of it - whilst as a rule of thumbs the blue stripes of START and STOP would have been needed to point to the left with a to-the-middle-of-the-tag orientation. Especially the color selection and sequence ordering of STOP seems to be the subject of such errors leading to decoding errors and needs for decoder workarounds for the field that effectively weakened the system. Even some early times advertisement materials exposed such flaws. Also its said that checksum labels had been wrong sometimes, and even the label set itself had some variations in respect to the imprinted number.

- = not used / reserved
white = white/black checker pattern aka checkered

The labels, also understood as data lines, each had two horizontal stripes that together represented a single symbol of information. The used colors for the stripes were blue, white, red and black. This does make up a total of 16 combinations where only 12 were used in the center area by just excluding black to be the lower color. For sensor reasons the white color was dimmed down by a black checkerboard so that they roughly met with the intensity of red and blue that were light sensed via a color filters.

The labels each are 5+3⁄4 inches (15 cm) wide and 1 inch (2.5 cm) high. With a 3⁄8 inch (0.95 cm) vertical gap between the labels realized a total height of 17.5 inches (44 cm). Labels could be affixed directly to the car side, but usually were applied to dark plates, which were then riveted to each side of the car.[2]

The labels were made from retroreflective plastic sheet that was coated with red or blue dye to provide distinguishable color filters. The retroreflective material gave a clear optical signal that could be read from a 9-to-12-foot (2.7 to 3.7 m) distance and easily distinguished from the other markings on the railcar. The white areas provided both a red and blue optical response to the reader, and were patterned with dots so that their brightness would be about the same as a red or blue stripe.

The start and stop labels were partially filled, so that the reader scanning beam would be centered on them before they were recognized. This ensured that the entire label was centered and had the best chance of being read accurately.

The labels are to be read from bottom to top:

• Line 13: check digit.
• Line 12: stop label.
• Lines 6 to 11: car number.
• Lines 2 to 5: equipment/owner code.
  • Line 2: equipment code
  • Lines 3 to 5: ownership code
• Line 1: start label.

The first digit of the equipment owner (line 2) marks the type of equipment: 0 for railroad-owned, 1 for privately-owned, or 6 for non-revenue equipment.

The car number is left-padded with zeroes if necessary. For locomotives, line 6 is the type of unit and line 7 the suffix number.

The check digit is calculated as follows: Each number digit is multiplied by two to the power of the labels's position minus two. Thus, the first digit (line 2) is multiplied by 1, the second by 2, the third by 4, the fourth by 8 and so on, until the 10th, which is multiplied by 512. The sum of all these numbers modulo is the check digit.[1]

The code on the caboose in the picture at top can be decoded as Start 8350199918 Stop 5. This means a car with equipment code number 8, ownership code 350, which lists this as a car of the Illinois Central Railroad,[3] car number 199918, with a check digit of 5.

A KarTrak scanner in Midland, Michigan.

The readers were optical scanners, somewhat like the barcode scanners used for grocery barcode items today. The scanning distances and speeds meant that the processing electronics needed to be state-of-the-art for its day. They were placed along the rail lines, often at the entrance and exit of a switchyard, spaced back from the tracks so that the labels would pass in the reading zone, 9 to 12 feet (2.7 to 3.7 m) from the scanner.

The scanners were housed in metal boxes about the size of a mini-refrigerator. They consisted of a collimated 100-200 watt xenon arc light source arranged co-axially with red and blue sensing photo tubes. The coaxial optical arrangement provided optimum sensing of the retroreflective labels. This optical source and sensing beam was directed to a large (8–14 in or 20–36 cm) mirrored rotating wheel that provided the vertical scanning of the railcar. The movement of the train provided the horizontal scanning. Although the system could capture labels at 60 miles per hour (97 km/h), often the speeds were much lower.

The scanner's analog video signals were passed to a nearby rail equipment hut where the processing and computing electronics were located. The first systems were discrete circuits and logic and only provided an ASCII-coded list of the labels that passed the scanner. These were forwarded to the rail operators for manual tracking or integration with their computer systems. Later reading systems were coupled with era minicomputers (Digital Equipment Corporation PDP-8s), and more elaborate tracking and weighing systems were integrated. Sometimes these included many railyard input sensors, for rail switch position, car passage, and hot wheel bearing sensors. Some of the more productive and thus longer-lived systems were installed in captive rail applications that carried bulk goods from mines to smelter, where the weight of individual cars loaded and unloaded tracked the bulk inventory.