Railroads Specify Creosote for Good Reasons

Stephen T. Smith, P.E.

As documented in the recent report (1) for the Association of American Railroads (AAR), approximately 95% of all new railroad ties are preserved wood, as opposed to non-wood products of concrete, steel, or plastic. Of the wooden ties purchased, 98% are either creosote or creosote-borate treated. Approximately 2% of wooden ties are copper naphthenate or copper naphthenate-borate treated. This paper explores the reasons why railroads continue to specify creosote preservative treated wood for their ties.

Wood

There are several reasons why railroads continue to use wooden ties to support their steel rails. Originally, back in the pre- and early 1900s, wood was the only option, as well as being readily available in North America, economical, and flexible. The introduction of wood preservation and recovery coke ovens, which made creosote widely available, added long service life to the advantages of wood over steel or concrete.

Experience, research, and innovation have revealed new and enhanced known advantages. Wooden ties absorb some of the shock of heavy rail wheels traveling the track, thus reducing wear and tear on the rails and trains. Wood does not conduct electricity, so the ties do not interfere with electric rail monitoring. Borate dual treatment with oil-borne preservative adds preservative into the tie heartwood and, thus, greatly extends the service of wooden ties in high decay regions.

Service Life

Service life of wood crossties is typically considered to average 40 years. Properly treated, wood crossties do not only fail due to deterioration from fungi and/or insects. The repeated mechanical wear caused by train loadings and rail components is exacerbated by biological deterioration. Eventually the rail systems cut into the wood ties and the spikes can no longer anchor the rails in place. Failing ties will cause derailments. Wood crossties must continue to perform under extreme weather conditions, exposure to decay organisms, and mechanical impact loading from high-tonnage train cars.

There are several experimental field-test methods to evaluate performance of the preservatives that protect wood against decay and insect attack. These methods include post tests (Table 1) and 2×4-stake tests, (Table 2). Also consider that the closer the sample size and the exposure are to real-life conditions, the more meaningful the results. Thus, the most reliable data of efficacy is actual rail performance of the wood crosstie. Railroads collectively have over a century of experience with creosote-treated wooded ties. In a 2007 article in CrossTies (2), service life of preserved wood ties was estimated to be 19 years for high humidity and a southern wet climate exposure, and high-tonnage curved track and up to 50 years for lower humidity, low tonnage, and straight tangent track.

The conclusion was that for most situations, wood crossties offered the lowest life-cycle cost. The introduction of borate dual treatment with creosote or copper naphthenate has improved service life in the high decay regions. (3)

In 1949, testing was initiated to compare many different wood preservatives’ performance in protecting southern pine fence posts of 4 to 5-inch diameters. The test included 25 posts of each preservative. Preservatives included creosote of many grades and types, copper naphthenate, and petroleum oils. Data from this study of preservatives applicable to railroad ties is summarized in Table 1.

Table 1: Post Test Results

This test was initiated in 1949 and was, in part, designed to help determine which creosote formulations or distillation cuts would provide the best performance. Based on results such as these and a similar test, the 1958 Cooperative Test, the AWPA standards for creosote were changed in 1978 to include less of the low temperature distillate and more of the medium residue material. (4) So, in Table 1, the line “Average of medium residue creosotes” best represents current AWPA Standard P1/P13 and P2 creosotes in use with an estimated service life of approximately 63 years. By observation of the various creosote results, one can also see the wide range of variability in results such tests yield. Thus, results should not be accepted as precise, but rather general indications of estimated service life.

Table 1 also includes a result for copper naphthenate, the other wood preservative typically used for railroad ties. While the estimated service life of 65 years is slightly longer than for the average of medium residue creosotes, it is, considering data variability, the same. In this case, it is also worth noting that the copper naphthenate is mixed with #4 petroleum distillate, which itself provides significant preservative value with estimated service life of 43 years. Current copper naphthenate treatment often uses #2 petroleum distillate as a carrier, rather than #4, so may not perform as well as indicated in this study.

The Forest Products Laboratory periodically publishes a summary of various wood preservative test results, the latest being the 2011 Progress Report. (5)  These tests are all for ground contact exposure. The stakes are driven into the soil in locations of high decay and insect (termite) hazard. Various test results are summarized in many different tables, each representing separate tests.

In Table 2, Progress Report results of tests using only 2” by 4” nominal by 18” stakes and of only creosote or copper naphthenate preservatives are summarized. It is noteworthy that even with about 50 years of exposure, some estimates of service life could not yet be made (shown as NYD) because some stakes had not yet failed. Since one purpose of the tests is to determine the optimum preservative retention for given applications, various retentions are tested together. For this review, note that for railroad ties of oak, hickory, or mixed hardwoods the retention of creosote is 7.0 pound per cubic foot (pcf) or refusal and for copper naphthenate it is 0.55 pcf or refusal (as copper metal). For southern pine, retention standards are 8.0 pcf for creosote and 0.060 pcf for copper naphthenate (as copper metal). (6) In Table 2, results with retentions close to the retention standard for crossties are highlighted with grey background for reference. Although many of the creosote tests have not yet determined the estimated service life, it appears estimated service life for these samples would be 30 or more years. Results for copper naphthenate are similar at about 26 years.

The conclusion from review of the summarized information in the tables is that both creosote and copper naphthenate provide good protection of wood in ground contact at the retentions specified by AWPA.

Performance Differences between Preservatives

While creosote and copper naphthenate both provide good protection from decay and insect attach, there are differences between them that should be considered. Some are briefly discussed below:

Weatherability

Both petroleum oil and creosote help to seal the surface of wood ties from water penetration. It is important to consider the volatility differences between #2, #4, and #6 petroleum oil. As water penetrates and goes through cycles of saturation, freezing, and drying, wood fiber is damaged. Petroleum oil, particularly #2 distillate, evaporates within just a few years. Creosote has a high residue and is less volatile in general than #2 petroleum oil. Thus, creosote tends to protect the wood from weathering better than certain types of petroleum-based preservatives.

Lubricating Properties

The lubricating properties of preservative help to maintain the flexibility and shock absorbing qualities naturally present in wood. As wood weathers, these qualities are reduced. Both creosote and petroleum oil help to reduce the loss of these qualities. However, since the petroleum oil generally does not last in the wood as long as creosote does, it is likely that creosote will maintain flexibility and shock absorbing capacity of wood ties longer than petroleum oil borne copper naphthenate.

Diluent Decay Resistance

Petroleum oil by itself evaporates or biologically decays. As shown in Table 1, posts treated only with #2 petroleum oil (used as a control sample) lasted about 8 years. As a comparative post treated with only the #4 aromatic petroleum oil (also used as a control) lasted 43 years. With the copper naphthenate added posts lasted about 65 years, it is not clear from this test how much of that performance resulted from the #4 residual petroleum oil carrier. As cited in the 1958 Cooperative Creosote Test (7) reduction in service-life performance is also documented where creosote is mixed with a #6 petroleum oil for the AWPA P3 Creosote-Petroleum Solution. In the 1958 Post Study, posts treated with creosote mixed half and half with #6 petroleum oil had an estimated service life of about 40 years, about one-third less than for straight creosote P1/P13.

Regulation of Preservative

While creosote is regulated by EPA as a Restricted Use Pesticide and copper naphthenate is not, the way that treating plants handle preservatives and ties is basically the same. Plant operators do not wish to contaminate soil or water by employing lax procedures or using leaking equipment. At the end of life, most ties, independent of preservative, are recycled for energy recovery via cogeneration plants, heat recovery combustion systems, or cement kilns. There is no practical difference in the way railroads handle used ties.

Disposal Costs

When wood crossties are removed from railroad service, most are either reused, if their condition warrants it, by a shortline railroad system or in lower tonnage track, recycled for energy recovery or disposed in landfills. These options have costs, but do not vary with type of preservative.

Conclusion

The railroad industry has more than a century of experience using creosote treated railroad ties. The vast majority of ties currently purchased are creosote and creosote/borate treated wood. Experience and testing continue to support the use of creosote preservation for wood ties.

While the railroad industry has much less experience with copper naphthenate, testing results and limited experience indicate that it is also a good wood preservative that provides a service life about equal to that offered by creosote. It is important that railroads recognize that the performance of copper naphthenate is likely to be negatively impacted when higher volatility, lower residue petroleum oil is used as the carrier.

Railroads must weight many factors in deciding what type of ties to use to support their rails. Should they use preservative treated wood or other products such as concrete, plastic, or steel? If wood, what type of preservative and with or without borate dual treatment? Many of the factors railroads need to consider are outlined above. For most cases, continued use of creosote preservative is favored.

Notes

1. Smith, S.T. (2019) 2018 Railroad Tie Survey, Journal of Transportation Technologies, 9, 276-286.

2. Zarembski, A. M. and Kondapalli, S. (2007) Development of Comparative Crosstie Unit Costs and Values, pp 17-18, CrossTies Jan-Feb 2007.

3. Amburgey, T. L. and Sanderes, M. G. (2009) Tie Dual Treatments with TimBor and Creosote or Copper Naphthenate, 20 Years of Exposure in AWPA Hazard Zone 4. CrossTies, pp. 20-22, Nov-Dec 2009.

4. Webb, D. A., McKinney, S. A., and Pfeiffer, R. G. (2010) Assay Creosote Extraction of Selected Posts from the 1958 Cooperative Test after 50 Years of Exposure as a Ground Contact Preservative. 2010 American Wood Preservers’ Association Proceedings, pp 242-248.

5. Woodward, Bessie M.; Hatfield, Cherilyn A.; Lebow, Stan T. 2011. Comparison of Wood Preservatives in Stake Tests: 2011 Progress Report. Research Note FPL-RN-02. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, p. 120.

6. AWPA, 2018. 2018 AWPA Book of Standards. American Wood Protection Association, Birmingham, AL.

7. Webb (2004) IBID.