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July 25, 2010

The AC - DC Railroad

A few weekends ago, service on Metro-North and Amtrak was thrown into chaos when two trains ripped down portions of the overhead caternary (power line). Trains were cancelled, weekend riders stranded.

Metro-North’s service in Connecticut is made all the more challenging by a technological quirk of fate. Ours is the only commuter railroad in the U.S. that operates on three modes of power… AC, DC and diesel.

On a typical run from, say, New Haven to Grand Central, the first part of the journey is done “under the wire”, the trains being powered by 13,000 volt AC overhead wires, or catenaries. Around Pelham, in Westchester County, the pantographs are lowered and the conversion is made to 660 volt DC third-rail power for the rest of the trip into New York. Even diesel engines must convert to third-rail, as their smoky exhaust is banned in the Park Avenue tunnels.

And there’s the rub: Connecticut trains need both AC and DC, overhead and third-rail, power pick-ups and processors. That means a lot more electronics, and added cost, for each car. While the DC-only new M7 cars running in Westchester cost about $2 million each, the dual-mode M8 car designed for Connecticut will cost considerably more.

So, some folks are asking… “Why not just use one power source? Just replace the overhead wires with third-rail and we can buy cheaper cars.” Simple, yes. Smart, no. And here’s why.

 There’s not enough space to lay a third-rail along each of the four sets of tracks in the existing right of way. All four existing tracks would have to be ripped out and the space between them widened. Every bridge and tunnel would have to be widened, platforms moved and land acquired. Cost? Probably hundreds of millions of dollars, years of construction and service disruptions.

 Even with third-rail, the CDOT would still be required to provide overhead power lines for Amtrak. That would mean maintaining two power systems at double the cost. We’re currently spending billions just to upgrade the 80-year old catenary, so why then replace it with third-rail?

 Third-rail AC power requires power substations every few miles, meaning further construction and real estate. The environmental lawsuits alone would kill this idea.

 DC-powered third rail is less efficient. Trains accelerate much faster using overhead AC voltage, the power source used by the fastest trains in the world… the TGV, Shinkansen, etc. On third-rail speeds are limited to 75 miles an hour vs. 90 mph under the wire. That means, mile for mile, commute time is longer using third rail.

 Third-rail ices up in bad weather and can get buried in snow, causing short circuits. Overhead wires have problems sometimes, but they are never buried in a blizzard.

 Third-rail is dangerous to pedestrians and track workers.
The idea of conversion to third-rail was studied in the 1980’s by consultants to CDOT. They concluded that, while cumbersome and costly, the current dual-power system is, in the long run, cheaper and more efficient than installing third-rail. This time, the engineers at CDOT got it right.

Not satisfied, some of the third-rail fans tried pushing bills through the Legislature in 2005 to study the replacement scheme yet again. More studies would have meant years of delay in ordering already overdue car replacements. Fortunately, the Legislature dispensed with these nuisance proposals quickly.

Doubtless, we’ll have further “wires down” problems in the years to come. Ironically, Metro-North’s 97% on-time record has made us come to expect stellar service, despite our ancient infrastructure. But in the long run, service will be faster and even more reliable by sticking with our dual-mode system.

1 comment:

StamfordNotes said...

That's very helpful - I was trapped on the train for 45 minutes last Wednesday and really didn't understand why until just this minute.