Thursday, January 24, 2008

Why light rail cannot work

I posted before on the reality that light rail doesn't work because, more or less, it has high profile and extremely high weight, and also is pretty much guaranteed to be empty 75% of the time. Now, here's why the darned things are "light" if they weigh a "mere" 50 tons, as do the Hiawatha Line carriages.

It all comes down to keeping the force vector between the wheels. More or less, if the center of mass, combined with lateral forces, don't point between the wheels, the carriage topples off the tracks. The designer therefore must cope with towing strains, centripetal acceleration, and the possibility that all the passengers go to one side of the carriage to see a pretty girl or a cow.

When the rail gauge is 4'8.5", and the width of the carriage is 12' or so, there is only one way to do this; make it far heavier than the likely load--generally 10x heavier. Of course, in doing so, you eliminate any efficiency advantages conferred by the low rolling resistance of steel on steel--it's lost in the energy needed to accelerate.

How to fix this? Use a wider gauge, and lower the center of mass, of course. When you do this, though, you more or less are required to build a road for your train.

And in that case, why not build a real road and use existing technologies like "buses" and "cars" on it? Vehicles that can climb, say, more than a 3% grade, don't need switches to change lanes, and don't need a roundhouse to be turned around?

Passenger rail was a great idea when the only efficient engine was the large steam engine. However, since guys like Benz, Diesel, Edison, Westinghouse, Tesla, and Ford came around, they've lost the market for a very good reason.


jroosh said...

Your example explains another phenomenon.

Al Gore is quite a bit wider than his footprint and carries much of his weight high and behind him.

This explains his propensity to fall flat on his ass every time he opens his mouth.

Bike Bubba said...

If only his fans could see past their bias to their hero's blatant hypocrisy....