More wire woes on the Green Line
By adamg - Tue, 07/21/2009 - 9:15am.
No Green Line service north of Government Center thanks to "a wire problem" at North Station.
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It's curious that the wire in the North Station section
is some of the newest on the Green Line (having been installed as part of the "SuperStation" project), but seems to have the highest failure rate of any of the wire on the system - based on the number of reported failures within the past year.
Unless "wire problem" is just the random excuse of the day to explain delays on that end of the line.
Defective pantographs, the
Defective pantographs, the device on the roof of a streetcar which slides along the wire to get power, can also cause "wire problems". When streetcars are crossing from one track to another to change ends, as they do north of North Station, more of the pantograph surface is making contact with the wires, increasing the chances that the pantograph could snag on the wire if there is any defect on the surface of the carbon insert that actually makes contact with the wire.
I don't know what caused the problem today, but I was near North Station outside around noon and saw that the "emergency wire crew" was ont of the MBTA trucks responding to the problem.
Anon - you are correct that a defective pantograph
usually results in a wire failure. However, it seems to me that the probability of a train developing a defective pantograph would be about the same regardless of where that train is on the system.
Therefore, this doesn't totally explain to me why Green Line wire failures seem to be most common in the North Station area as of late.
pantographs with defects
pantographs with defects have a greater chance to snag when they are going through special work (like North Station) where more of the pantograph surface is coming in contact with wire (and there is more criss-crossing wire to come in contact with)
Again, you are correct. However, given that defective
pantographs can easily get messed up on special work, why haven't there more wire failures at places like the interlockings at Copley Junction and Kenmore, which have similar special work (and see more trains through the area than the North Station turnbacks do) and whose installations are considerably older than North Station?
The number of wire failures in the North Station area that have occurred recently, as compared to the rest of the Green Line, suggests to me that there is a basic flaw in the design of, or the materials used for, the wire in this area.
And had they properly built the North Station turnback as a loop instead of stub end tracks, it's unlikey we would even be having this discussion. But that's another matter for another day.
The special work at North
The special work at North Station is more complex than the other locations you mentioned and more complex than any other turning location. Its two pocket tracks with multiple switches from each running rail. Lechmere, Government Center, Park St. , Lake St., Cleveland Circle, Heath St. are all simple loops. Riverside is a simple cross-over. No place on a revenue segment has as much pantograph/wire interaction as the North Station turn-back, thus the greater chance for problems that might not be related to the orignal installation at all, but rather a greater percentage of potentially defective pantograph surfaces making contact with wire in ways they can't at other locations.
OK, I concede the point.
Seriously, anon, this has been an interesting discussion.
Thinking more about the track layout at North Station turnback, you are right that it is complex enough to require some very special special work. See previous comment about possible poor design.
Which leads to the next question: Why are the pantographs on Green Line streetcars apparently so prone to causing wire failures?
Oh wait, I already have the answer to that. Because Danny Boy considers any preventative maintenance - like replacing the carbon pickups on the pantographs before they completely wear out - to be a waste of money.
'nuff said.