When Concrete Fails: How a Humble Rubber Pad Keeps Rail Travel Safe




When you think about it, using concrete for railroad ties makes a lot of sense. Concrete doesn’t rot, it can keep a heavy steel rail stable, and it’s more environmentally friendly than the alternative (creosote-soaked wood). Plus, it doesn’t catch fire.

Image: “Concrete sleeper 1638” by Walter Siegmund

So why don’t we use it everywhere?

Well, outside the US, we often do. “In Europe and Japan, where rail transport is arguably a higher priority, concrete [rail ties] have been gaining ground since the end of World War II,” writes Chris Lo on this website devoted entirely to rail ties. “In Australia, concrete is used for most railway [ties] and in the UK, rail operator Network Rail replaces 200,000 wooden railway [ties] with concrete ones every year.”

There are a few cases in the US where concrete has been used too, although not always successfully.

When concrete fails

As you know, different materials often call for different engineering. Concrete is rigid and it doesn’t absorb vibration well like wood. And the worst part? All it takes is about an inch of wear on the tie to twist the rail out of position. When that happens, it can send a moving train right off the tracks.

That’s what happened in 2005, on a curve outside of Pasco, Washington, when an Amtrak passenger train derailed as it crossed over concrete ties. The National Transportation Safety Board determined the problem was caused by abrasion on the tie underneath the rail, the result of years of trains passing over.

Check out these images from the accident brief.

 

 


The takeaway? Tie pads matter

And that brings us to the focus of this post. The humble tie pad. Seen in the photo above, the tie pad protects the tie from vibration and impact from the rail as heavy trains pass over. It also prevents corrosion on the rail and erosion on the concrete. Without that tiny square of rubber, most concrete rail systems could not hold up for long.

The tie pad in the Amtrak derailment was not blamed for the abrasion on the rail, necessarily. But it illustrates just how important it is to get this one piece of the system right. According to Sanrok Enterprises, a provider of rail products, services, and solutions (and a PTC Creo Parametric user), these pads have to be carefully engineered with regard to design, materials, and vulcanization “so as to obtain the fine balance between resilience and damping.”

The pad itself has to incorporate springs, of course, to respond to varying loads. The shape should be optimized so that the ratio of the loaded area to the force free areas are balanced. And the material must be produced completely free of dust particles to keep abrasion minimal.

Sanrok has even gone one step further with its products, devising a pad that keeps itself clean by directing wind and rain in such a way that the elements wash away debris from the environment.

Meet the Sanrok rail pad

Now that you know much of what the pad needs to do and why it’s so important, here’s one of Sanrok’s deceptively simple-looking solutions:

 

While rail pads are plenty important where they’re used, they’re about to get even more critical to safe travel in India (where Sanrok is based). That’s because axle loads and speeds are being increased to 32.5 tons and 180 km/hr, respectively, on that country’s railways. In other words, those few square inches of rubber have become even more crucial to preventing accidents like the one in Pascoe.

PTC in the mix

The Sanrok team says PTC Creo Parametric, our 3D CAD design software, is helping throughout the company’s product line, which also includes air brake accessories and twist locks that hold containers on rail cars. “PTC Creo Parametric has enabled us to reduce our design time by approximately 20 – 30%,” says Girish Kathuria Chief Operating Officer & Head of Engineering. ”We can make radical design changes much faster.”

Read more about PTC customers and the remarkable engineering challenges they conquer every day, all over the world. Visit our Case Studies page. And then sign up for our newsletter, PTC Express, to meet even more PTC customers like Sanrok each month.

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