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August 1, 2000
DART Rail Construction Update
"Seamless" Track Keeps DART Light Rail Smooth and Quiet
If you're at the train station and it's very quiet, you can almost hear it. It's a type of singing sound, and it's the first thing many people notice when a DART light rail train is coming.
The seamless track carries the sound of wheel on rail several feet ahead of the train. It's a unique and quiet sound and it's only possible because of a special type of track.
The trains coming to Richardson, Garland and Plano look like trains. They have air horns and run on tracks. Still, DART's yellow light rail cars don't "click and clack" over the track like we expect trains to when they move. Because light rail trains run on continuous welded rail, a type of seamless track, they "sing" as they approach and "whoosh" as they go past.
During most of rail's history, individual pieces of track were connected at joints by bolts. The idea of replacing jointed rail with continuous welded rail originated in the early 1920s by electric street railways, and over time has gained popularity.
Maintenance is the main problem associated with traditional jointed rail. Track crews have to re-tighten bolts frequently, identify cracked or broken bolts and rails, replace worn joint bars and refurbish battered rail ends. As a result, many companies have begun using continuous welded rail to reduce recurring maintenance problems and save money. Customers benefit since trains running on continuous welded rail experience less vibration and the noise that is associated with jointed rail.
"Light rail has taken off in North Texas and is a world-wide phenomenon. Aside from being excited about giving the public another transportation alternative, we're thrilled that the use of continuous welded rail for the track enables us to provide a greater sense of security and comfort to our customers, while minimizing the agency's maintenance costs," said DART Assistant Vice President, Facilities Engineering Rick Brown.
Track-laying is state-of-the-art
DART's light rail track system begins in Pueblo, Colorado -- home of the Rocky Mountain Steel Mill -- where 80-foot long rails are manufactured and transported to a nearby welding plant. At the welding plant, the rails are electric-flash butt-welded -- a process that fuses the rail ends together by passing current between the rails -- into 720-, 1,200- and 1,440-foot welded rails. The flexible welded rails are then loaded onto multiple rail cars and shipped to Dallas.
Before laying the track, construction crews must put subballast -- a type of gravel -- in the roadbed. This is followed by ballast -- crushed rock -- that anchors the track and allows water to drain through. Next, ties -- supports to which railroad rails are fastened to keep them in line -- are added. The ties keep the rails spaced apart at the proper distance, which is also referred to as gauge. Together, the ties and rail fastenings keep the track at 56-1/2 inches -- the proper gauge or width.
Once the ballast and ties are laid, the welded rails are placed on the crossties. More ballast is poured and then the track is surfaced and aligned by a tamping machine that picks up the track and compacts the ballast under the ties at the same time it levels and lines the track. A field welding process is then used to connect the rails into continuous welded rail. Once this process is complete, the tracks are ready for the trains.
"At one point in time, the train was the way to go," said DART Assistant Vice President, Facilities Engineering Rick Brown. "However, as other transportation means became available, people began to steer away from rail transit due to the convenience of the automobile. Now, it's coming back."
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