Tuesday, January 13, 2009

Sand Traps

In and around San Francisco, there are places where LRVs stop and you'll see lots of sand. For a while I wondered where it came from until I talked to some folks who told me that every LRV has a sandbox under the seats behind the wheels to gain traction going up hills and on rainy days. Today in the Rocky Mountain News. One of the LRT mechanics discussed the sandbox:
It's for traction, but it's pure, high-grade sand, not salt or chemicals, according to a reader who's a mechanic on light-rail cars. "When the propulsion system detects either sliding while braking or spinning while powering, sand is dropped to increase traction between the wheels and the rails," the reader wrote.
You can see the sand on the ground here in the photo below. It's the best one I could find. (Note to self, take better pictures of the little details)



jfruh said...

This fall in Baltimore, a new electronic braking system on the LRVs, combined with leaves on the track being ground to goo by the wheels, resulted in too-swift braking that caused slides and potential wheel flattening, and shut down half the system for more than a week.

They supposedly tweaked the brakes to fix it; maybe a lower-tech fix like this would have been in order?

Bob R. said...

On the Portland LRVs, you can clearly see the sandboxes with a little window to indicate the sand level, and you can hear them operating... I don't know how that compares with the Breda cars in San Francisco.

Hearing them operate, for new passengers, especially when one is seated directly above one, can be a bit disconcerting -- but not like flying for the first time and hearing all the hydraulic noises as the wing flaps/trim are adjusted. :-)

Justin said...

jfruh: Leaves on tracks is a HUGE problem. I know in the UK, they spending a lot of money trying to fix the problem. Apparently, there is a laser that can burn away the gunk that forms on the tracks.

arcady said...

In the UK, their trains don't have sanders. That one simple measure could solve a lot of their problems. It's strange that what is common practice on all US rail operations, from freights to commuters to light rail, is completely unheard of in the UK.

Pedestrianist said...

@Bob R,

The Breda cars do indeed have a window in the car. I imagine it's important so the technicians can tell if they need to top off the sand.

njh said...

re leaves, I've wondered whether a scrapper or grinder ahead of the leading wheel would be a more effective solution. Another possibility would be very high pressure water.

ben in sf said...

San Francisco's cable cars also have sand boxes, to help the wooden track brakes grab the rails better.
The track brakes sit between the wheels above the rails and lift the car such that it slides along on the wood blocks. They have to be replaced rather frequently. (I forget whether they have brake shoes that grab the wheels themselves).