Wednesday, January 30, 2008

Why is BART Broad Gauge?

BART tracks have the rails 5' 6" apart, as opposed to US standard gauge, which is 4' 8.5"--a historical accident, maybe, but nevertheless a standard, used by our national railway network and all other new transit systems.

This is a major obstacle to physical merger of BART with other regional transit networks such as CalTrain and SF's and SJ's light rail.

What was the reason for this choice? Over the years, I've heard quite a few:

  • A broader gauge allows higher speeds (but note--BART tops out at 80mph, while TGV's and Bullet trains go 3x as fast on standard gauge track).
  • It makes room for larger electrical motors (but BART car trucks are build with frames inside of the wheels--outside framed, standard gauage trucks would probably leave just as much room).
  • MetroRiderLA's Wad mentions a reason I hadn't heard before: a broad gauge would make trains more stable, and able to run across the Golden Gate Bridge. But he notes that Marin had opted out of BART long before the system was built, or even designed.
  • Compatibility with Indian railways (just kidding, but it makes as much sense as anything else).

None of these really add up. My best guess for BART's eschewing of standard gauge is just:

  • Not invented here.

You can find out all you want to know about railway gauges and more in this Wikipedia entry



Pantograph Trolleypole said...

When BART was designed they brought in aerospace engineers to develop a new kind of subway. Basically they eschewed everything the transit industry learned over the previous century for a more space age application. The broad gauge was supposed to allow the cars to be wider and therefor run more smoothly and have better interiors. While this might have been achieved, they also created a situation where every new extension is a trial in engineering for firms that have built standard gauge but must familiarize themselves with new systems.

Anonymous said...

A reason I once heard for the exotic gauge was as to make interoperability decidedly impossible. Apparently there was some fear that the system could be one day the subject of a (federal?) deregulation policy and opting for fantasy metrics would impose physical barriers to any attempt to hijack the system. Or make it at least highly cost prohibitive. Not sure if the former is true, but the latter has proven itself.

Raffzah said...

A wider gauge makes a smoother ride regardless of speed - it just lets your head bob less.

It's all about the alignment of your rails. It one rail is about 1 inch higher than the other, your car gets tilted by 1 degree (roughly) on standard gauge, while on colonial(indian) gauge the same missalignment translates to only 0.85 degree, or about 1/6th less, wich is in line with the 1/5th wider gauge ... isn't the litte trigonometry we remember handy :))

So when building a new system, one would want a gauge as wide as possible, and BART had the comfortable situation to be a large scale project. When you design a new system with like 5 miles of track and 4 cars, you rather stay to standard equippment, cause no manufacturer will build to your specs and offer a good price. But when you're allowed to build a 100+ miles system with 500+ cars, you got all freedom in design you want.

Choosing colonial spur was in fact, unlike Pantograph suggests, a rather conservative decision. That way, they could use all standard metrics for Construction.

Also I never heared (at least in Europe) of companies to have trouble in adepting to different gauges. Over here they do standard or meter or broad, or whatever comes allong and get paied :)