In the last article I provided some basic information on types of ties, lifecycles, and how many ties are used annually. This article is going to focus on the different types of grades, why each one is important, and how the grade is determined. When a load of ties is delivered to a plant they are going to be inspected and sorted based on size and type. There are four main categories that ties and timbers fall into: crossties, switch ties, bridge timbes, and specialty products. Each category has its own grade breakdown, based on the purpose of the tie or timber in track.
When a train passes over a crosstie part of the job of the timber is to distribute the weight. The weight at the axel is distributed from the wheel to the rail, then to the rail plate, then to the tie, the ballast, the sub ballast, the grade, the sub-grade, and ultimately to the ground. If everything is working as it should a standard rail car, weighing 286,000 pounds would have four axels. By calculating the size of the rail seat, the rail plate, the surface area of the crosstie, the interaction with the ballast, then the sub-ballast, the grade, and sub-grade, the ultimate impact to the ground is 12 psi. In addition to this, the wooden tie absorbs the weight and distributes it evenly under the track. The wood naturally bends and flexes under the two point of impact and returns to it’s original shape and location once the load is removed. Over the years railroads have used 9′, 8’6″, and 8′ standard rail ties. Today the majority of freight railroads have settled on 8’6″ as the ideal length. Figure 3 shows what happens when a tie is too long or too short, along with the load evenly distributed for the correct length for each circumstance.
In Figure 3, the span on the left is too long, causing narrow gauge, while the span on the right is too short causing open or wide gauge. Finally, the tie illustrate at the bottom of the Figure creates an even load distribution and maintains proper gauge. What happens when the gauge is wide or narrow? This creates an environment where the train wheels can come off of the rail head and result in a derailment. There are several failure modes for ties having gauging issues. Rail cut or physical exhaustion are the two leading causes. Rail cut is when the tie plate physically cuts down into the face of the tie. This can create wide or narrow gauge issues.
In contrast, plate cut occurs most often in a curve where the horizontal force of the train passing by can create movement of the plate, eventually wearing a low spot into the wood surface. This could also occur due to a weakened spot caused by rot where the tie no longer has enough strength to hold up to the weight of the trains passing by.
There is an extreme amount of engineering that has gone into the dimensions, length, and attributes of each grade of tie. This has been developed over many years by organizations such as American Railway Engineering and Maintenance-Of-Way Association (AREMA). Specifically, AREMA Chapter 30 has specifications on Grade and Industrial Grade Tie criteria. In addition to these criteria some railroads have their own specification. These specifications are generally all similar to AREMA but have slight differences that each railroad has requested over many years. In recent years most specifications for inspecting ties have worked their way back to the AREMA criteria.
This criteria is looked at by the Chapter 30, Sub-Committee 2, Section 3 committee. This committee is made up of a variety of industry individuals representing end users, producers, chemical companies, and other suppliers. For any change to be made the sub-committee must be in agreement and then every voting member of Chapter 30 gets the opportunity to approve or deny the change. Almost all changes are voted for with a unanimous vote.
Now to grading the ties themselves. The first thing to know is that the area where the plate will set is called the Rail Bearing Area (RBA). This area gets special attention and has stricter specified limitations on the defects. The RBA needs to be able to have enough area for a plate to set on squarely, set flat, and be spiked or screwed into place. The spikes or screws need to remain tight for the lifecycle of the tie. The ends and middle of the tie are for general strength, so the specified defect limitations can be relaxed in these areas.
There are many railroads that have their own specification. These are generally pretty similar with some minor differences. For this reason prior to settling on which specification to follow you should reach out to your local procurement person or tie company to discuss the options. For this paper I am going to focus on the AREMA/RTA Specifications. These two are meant to mirror each other. The following chart shows the differences between Grade Tie and Industrial Grade Ties.
So, what does all of this information mean? Ultimately, every tie has a home. Each individual railroad or contractor has guidelines that they have to meet. Some railroads choose to only use 7″x9″, while others use up to 20% 7″x8″ ties in their main line track (allowable per FRA). Many roads will use a blend of grade and industrial grade, even 7″ and 6″ ties depending on where they are going. Main line, passing track, sidetrack, industrial siding, and even zoo or amusement park trains. The conclusion to this article is that every tie has a home. By cutting tie logs to minimize defects in the RBA you will get better tie grades. This maximizes the use of the natural resource.
