Questions Regarding Track Construction

I commute on the access road between 66 and 495 every day, so I've been able to see as the construction has progressed. As I creep along in the afternoon, trying to join 66, I've wondered about a couple things I've noticed. My apologies if these have been discussed here elsewhere, but I was unable to find them.

1. Why in some places is the track protected by a rather standard chain link fence, but in other places it has a hardened wall of concrete slabs between steel pillars? Is this noise abatement? Or I was thinking perhaps it's placed where there might be a greater chance a car could leave the road? And why are the segments of fence all grounded together? In case somehow the fence comes into contact with the energized rail?

2. What is the purpose of the boxes strung along the fence and connected together with a thick sheathed cable? My friend is guessing they're intended to detect derailments or something else which hits the fence, but I don't know.

Thanks!

"Roscoe129"
I commute on the access road between 66 and 495 every day, so I've been able to see as the construction has progressed. As I creep along in the afternoon, trying to join 66, I've wondered about a couple things I've noticed. My apologies if these have been discussed here elsewhere, but I was unable to find them.

1. Why in some places is the track protected by a rather standard chain link fence, but in other places it has a hardened wall of concrete slabs between steel pillars? Is this noise abatement? Or I was thinking perhaps it's placed where there might be a greater chance a car could leave the road?

Noise abatement. They can also be found along the E route Green line between Collage Park and Greenbelt and as I recall along segments of Suitland Parkway on the southern F route Green line.

And why are the segments of fence all grounded together? In case somehow the fence comes into contact with the energized rail?

All of WMATA's infrastructure is electrically bonded and grounded, In the case of the security fence It is to direct lighting strikes to ground, in the case of the rest of the infrastructure is to direct stray return traction current to ground to prevent electrolysis induced corrosion.

2. What is the purpose of the boxes strung along the fence and connected together with a thick sheathed cable? My friend is guessing they're intended to detect derailments or something else which hits the fence, but I don't know.

Those "thick sheathed cable", actually flexible watertight conduits and boxes on the fences are part of fence penetration detection system. Where Metrorail shares a right of way with a highway or railroad those conduits and boxes are strung along the security fence. In the event of a fence penetration by a car or truck along a shared highway right of way of a railroad car or locomotive along a railroad right of way the conduits will breakaway from the boxes, opening the circuit and sending an alarm to Central Control. Where the fence penetration detection system is along a railroad right of way the alarm is also sent the railroads central dispatching center.

I can recall at least 6 incidents where the security fences has been penetrated resulting in an alarm. 2 on VA I-66, 1 in Falls Church and 1 west of the Capitol Beltway, both were cars crashing through fence. 2 CSX derailments north of the Takoma station and 2 south of the Silver Spring station, 1 was a runaway car crash and the other was a vandal crashing a stolen backhoe into the fence. All happen at night during none revenue hours with the exception of the runaway car crash in Silver Spring, that one happen on a weekday before PM peak. No Metrorail trains were in the vicinity when the penetrations occurred.

Concrete wall for noise reduction is a joke. The best solution is material to absorb the sound.
Someone has a patent on walls with pockets; the idea being to promote plant cover such as vines that does the job.

"farecard"
Concrete wall for noise reduction is a joke. The best solution is material to absorb the sound.
Someone has a patent on walls with pockets; the idea being to promote plant cover such as vines that does the job.

The walls are not intended to absorb the sound they are there to reflect it away in the same way sound walls along highways reflect traffic noise away.

The problem with planting vegetation is the need to regularly trim it back to suppress the accumulation organic material. You don't want to contaminate the track ballast with organic material as it inhibits proper drainage.

Thanks very much!

So why do noise abatement walls need to be so heavily constructed? It appears that they're constructed of heavy concrete blocks between large metal posts. I saw them constructing them and they needed a hoist to lift the concrete blocks into place.

And I wonder how they decide who gets noise walls and who doesn't? The track on 66 east of where the line splits off and heads out the access road has no walls at all, yet there are houses very close to 66. I wonder if it's like what I've always heard about the walls on the freeway; that it's the neighborhoods that lobby hardest that get the walls.

"Roscoe129"
Thanks very much!

Your welcome.

So why do noise abatement walls need to be so heavily constructed? It appears that they're constructed of heavy concrete blocks between large metal posts. I saw them constructing them and they needed a hoist to lift the concrete blocks into place.

2 reasons, sheet metal barriers tend to act as sound board amplifying various frequencies, concrete does no corrode like sheet metal.

And I wonder how they decide who gets noise walls and who doesn't? The track on 66 east of where the line splits off and heads out the access road has no walls at all, yet there are houses very close to 66. I wonder if it's like what I've always heard about the walls on the freeway; that it's the neighborhoods that lobby hardest that get the walls.

VA I-66 from just east of Great Falls Street to the Dulles Connector Road is in an open cut. The banks of the open cut act as a sound barrier. The abutments and the flyovers have sound barriers on them because they rise to a level that is above the open cut.