Comment by SECProto | Hacker News Reader

Comment by SECProto | Hacker News Reader

Parent Full thread

Great video. Most neat parts for me:

- Removing the ballast from a section at a time to do the cleaning means that the rails and sleepers are floating in the air (~1:30 to 2:35)

- They're doing all this work, including transferring waste to another train, underneath catenary wires. low clearance!! (3:40 to 5:20)

Also important to note, they aren't so much washing the stones (though that may be part of the process), they're sieving (grading) the stones to remove everything that's under a specific size (looks like they're removing everything that passes a ~25mm/1" sieve, but scale in a video can be deceiving). Rail ballast is very uniformly graded, i.e. every piece of aggregate is about the same size. Vibrations over time will cause edges of the stones to chip and wear down, with these chips contaminating the ballast. It also leads to rounding of the aggregate, so there's less interlock between the particles.

The sequencing was fascinating!

I'd never considered how much "it helps to have rail, to replace rail" impacted the process -- but it makes sense as everything is incredibly heavy! (Rails, sleepers, aggregate)

From that perspective, of course you'd excavate and clean the track ballast while you're also on the same track. And same with sleeper and track replacement.

We don't see any tank wagons, and there would be no other possible water supply, so I don't think they are washing the stones. Given it gets rained on anyway, I doubt there would much benefit.

reminds me of how specific they were with crushed stone sizes while developing the first roadways for cars

I don't know much about the earliest car roadways, do you have a link with some info?

Granular gradation is still a major part of modern roadway construction, there are just certain standards that are very typical so they get used without too much thought required. For some applications a visual inspection is enough, but there's generally some standard proportion of sieve analysis required (eg. 1 test per 10000tonne produced), along with proctor tests (to determine maximum dry density) and frequent nuclear densitometry. I've even seen loads of aggregate get rejected due to the large aggregate being too oblongated.

When you get into concrete or asphalt mix design, the aggregate standards can be even more strict - chemical makeup of the stone can be important to avoid alkali-silica reaction; minimizing fines increases the strength of asphalt mix but creates a tougher riding surface; etc etc etc.

See https://en.wikipedia.org/wiki/Macadam#McAdam's_methods, but it's the first roadways for carriages, not cars.

> A workman could check the stone size himself by seeing if the stone would fit into his mouth.

Reply on news.ycombinator.com