Caltrain Vibration and Volatility Analysis

https://www.nathanblack.me/post/caltrain-vibration-and-volatility-analysis

by mmcclure • 7 years ago

76 63 7 years ago

Hansenq• 7 years ago

Thanks for doing this analysis!

After riding other transit in the US (NJ Transit, LIRR, Amtrak, BART, misc Subways), high speed rail in China and Japan, and trains in Europe; Caltrain stands out to me as the most jerky train I've been on. Often times high speed rail in East Asian countries are so smooth that you can balance a coin on its edge without falling. Comparing apples to apples, even NJ Transit and LIRR are noticeably smoother than Caltrain. It would be interesting to see what the difference is between systems.

Looking at data from a few different trips, Caltrain does a good job subjecting passengers to no more than 1g of roughness in any direction.

This at first to me seemed like a low bar (relative to other trains out there), but then I forgot about the sudden jerks and direction changes the SF Muni and NY Subway often subject passengers. 1g in any direction I suppose is good for subways, but still poor for trains overall.

Unfortunately I doubt Caltrain's current Electrification project won't solve any of these issues, given that it seems like more issues with the track, not with the power source. Perhaps the new trains might help?

kevin_thibedeau• 7 years ago

Not as hot in the summer for east coast commuter rail.

nhf• 7 years ago

Much of the side-to-side jerking is a combination of the poor suspension on the trucks (bogies) of the older Gallery train cars and fast running over track switches. The Gallery cars are the ones that look like steel toasters.

If you take a ride in one of the "newer" Bombardier cars (the octagonal ones) the ride is much smoother and the bumps are almost imperceptible with little swaying. I'd imagine the new trains post-electrification will also be much better on the suspension front.

carlob• 7 years ago

I'd imagine the new trains post-electrification will also be much better on the suspension front.

So right now this is still Diesel?

inferiorhuman13• 7 years ago

I imagine there's also quite a bit of hunting / side-to-side movement as well. BART tried to solve this with flat wheels, and after forty years of complaints came to the conclusion that flat wheels wear the track down more and cause significant noise. They've since cut a conical profile (like most rail operators) into the wheels under the guise of reducing noise. I can't hear much difference but the side-to-side motion is quite a bit more dramatic than I remember.

It's been almost a decade since I've ridden Caltrain with any regularity but I don't remember being particularly enamored with how the Bombardier cars rode. If anything they rode firmer than the Nippon Sharyo cars over the bumps but had less lateral movement as a consequence.

I remember there being quite a bit of variation (there are three distinct batches of them after all) – something you used to see with Muni buses. The German ones (Neoplan) touted their complex (independent) front suspension, and there was a huge variation in ride comfort with some being downright scary. The Hungarian (Ikarus) ones with their simpler solid axle up front seemed more consistently comfortable.

ZanyProgrammer• 7 years ago

Electrification will involve entirely new train sets, so it’ll go a very long ways towards fixing things. Electrified engines aren’t going to be pulling the older cars.

brian-armstrong• 7 years ago

This article is somewhat interesting but it appears to be a not so subtle advertisement for a trading platform.

mmcclure• 7 years ago

Really curious where you're getting that impression.

I like rhythm and vibration and I trade financial market volatility at sha.capital.

This is the only mention of anything associated with his work, and it's not even hyperlinked. It's a fund at that, not a platform, and he doesn't even make an attempt at any kind of solicitation in the post. If this is an attempt at advertising it's a pretty poor one.

statico2• 7 years ago

I’m glad somebody tried to quantify this. Here’s the rocky ride when taking a late Caltrain bullet from north to south (somewhere before Palo Alto): https://imgur.com/gallery/L3LRcld

Symbiote4• 7 years ago

That's insane! How fast is it going?

I don't think I kept it, but I now understand why it was requested by my friend: I took a video of a thick coin (£1 perhaps, 2.8mm thick) balanced in the train from Malmø to Copenhagen.

I can't remember how long it stayed balanced, at least a couple of minutes, but something like a full wine glass would be fine for the whole journey.

Edit: here's a video of a coin balanced for over 20 minutes on a Chinese HSR train: https://www.youtube.com/watch?v=fumYdO9XknE

inferiorhuman13• 7 years ago

Caltrain tops out at 79 mph or about 127 km/h, so most likely 100 km/h or slower.

short_sells_poo• 7 years ago

First I thought "This is going to be a case where a train geek obsesses over a slightly bumpy ride". Then I watched the video and my jaw dropped.

I have no words honestly. 150 year old cog wheel trains in the Alps offer a better ride than this.

Honestly that was atrocious. I can't believe people put up with it. That's a roller coaster ride, not a commuter train. With that sort of movement, you can't eat, drink or do any work. You are lucky to be able to have a nap.

statico2• 7 years ago

More info: This isn't what the ride is normally like. This was an express train in April, 2019, probably 6pm or 7pm, going south from San Francisco.

Because it's an express the train gets up to some high speeds, and this lateral wobbling starts to really get uncomfortable in the long stretches after Hillsdale or so.

Also, this is the balcony, which amplifies the wobbling. The lower level are much more stable, and I switched to sitting in them shortly after taking this video despite the cramped leg room.

And this was probably the car closest to the engine, like the author of this article said to avoid. I usually sat up there because it was the least crowded.

inferiorhuman13• 7 years ago

Caltrain tops out at under 80 mph, that's not really high speed by contemporary standards.

cameldrv• 7 years ago

The deviation from a normal distribution is really interesting!

blululu• 7 years ago

This is fun. Interesting analysis. A few quick thoughts:

Why the focus on Jerk/Volatility instead of Acceleration/Force? The energy imparted to the passengers is proportional to the forces. Is there any reason to suppose that Jerk is more important to the experience of discomfort (or mechanical wear for that matter)?

The analysis notes that the distribution is not normal, but it does not explore the what the underlying distribution might be instead. Given that the train's velocity is constantly accelerating/decelerating it seems unlikely that it would be a single distribution. However it may be the case that the observed variations are the result of a superposition of normal distributions. A Fourier transform could maybe helpful to explore these aspects.

nomel• 7 years ago

Acceleration/Force is easily compensated for, if its constant. For example, taking a turn or coming to a gradual stop is completely comfortable. Only the change is uncomfortable.

Gibbon1• 7 years ago

Jerk is what's unpleasant and annoying. In machine design it's something that is carefully dealt with since it has a strong effect on wear. See cam design.

0xFFFE• 7 years ago

Just something I noticed [there is a plate near the doors], cars were built in 1984 and refurbished by Alstom few years ago.

supernova87a3• 7 years ago

I had wanted to do an analysis like this too -- had even recorded the trace of acceleration using iPhone during rides. But the main thing I thought would be useful would be to overplot the acceleration levels of other train systems in the US and Europe/Asia for comparison.

Caltrain is simply an embarrassment to ride and realize this is what the center of global tech has to put up with. I swear a visitor from any other country with a reasonable rail system would ride Caltrain and think the train was about to shake itself apart. (Yes I realize the old silver gallery cars are especially bad while the Bombardier ones are better)

Even if the gross level of shaking were resolved, Caltrain and most other trains we put up with here are so incredibly loud compared to other more advanced systems. You go to Germany, and even their subways (subways!) are so quiet as to feel self-conscious when you're talking on them. And in the meantime BART -- despite being designed well into the 2nd/3rd generation of US transit systems -- screeches along due to poor rail maintenance.

ummonk3• 7 years ago

Funny, I hate the BART screeches so much that I consider Caltrain great in comparison. Guess I'm in blissful ignorance of what is possible. Hopefully the switch to electric trains in a decade will make Caltrain better.

supernova87a3• 7 years ago

Well actually, I've been told that the problem of rail noise comes from improper maintenance of the rail track, not the motive nature of the cars themselves.

Rail requires periodic grinding to be maintained properly. When you see rail and wheels riding "flat" on each other, that's actually not how it's supposed to be -- that kind of maximum contact actually produces the squealing because the surface area has lots of opportunity to make friction-induced noise.

Supposedly, when you see a rail that's completely shiny on the top surface from being worn down, that's a sign that it hasn't been properly maintained.

A properly ground rail and wheel actually is only interacting on a very small edge of contact -- and this is what produces a quiet ride. A properly maintained rail will be quieter than almost any other mode of transport (noise from the contact surface). It takes active rail grinding though.

So actually, just a change to electric cars will not maintain quiet operations (although in the beginning I'm sure the newness of it will help). BART/Caltrain actually need to conduct the rail maintenance in order to keep it quiet.

Unfortunately, whether it's due to budget priorities being elsewhere, or an indiscriminate customer who puts up with this, unless that aspect changes, we'll be back in the same place. I don't even know whether the conductors -- the ones who would be among the first to notice such issues -- are even trained to understand this.

I encourage anyone to go ride the high speed rails of Germany, France, Italy, China, Japan, Korea, to understand what properly maintained rail is capable of feeling like. Or even the U-bahn in Munich, Frankfurt, too.

scrumbledober• 7 years ago

I recently had a conversation at work where I commented how great Caltrain was since I used to commute on BART. My foreign-born coworkers all laughed at me. I look forward to riding some nicer train systems in the future.

privateSFacct• 7 years ago

Yep - having travelled a lot in Europe it is crazy how quiet some trains there are. Some just have a very light tick tick sound

mc32• 7 years ago

Yep these are built by Rohr, Alstrom or Bombardier. People who build rolling stock.

The issue is due to insufficient wear leveling, the tracks cause noise (the non independent (fixed) axles cause small wave-like wear on the track surface in track turns which in turn cause the screeching.

The fix is independent wheels, conical wheels or more frequent leveling of tracks.

CaliforniaKarl• 7 years ago

What would be interesting next would be to see what the track geometry is like in the area of & around the worst events.

Something like what you’d get from this: https://youtu.be/Ite0l4uJ_fM?t=3m55s

Animats• 7 years ago

He's seeing mostly the suspension problems of the older cars, not the track quality. Caltrain replaced all their track about 15 years ago, going to concrete ties and welded rail.

Caltrain uses a track geometry car twice a year to measure the rail geometry.[1] Those also measure vibration. The Shinkansen system in Japan does that once a week.[2] That's a very good system but extremely high maintenance. The Shinkansen lines have track maintenance every night.

There's a whole cult thing in Japan around "Dr Yellow", the track inspection trains. They're special-purpose 7-car Shinkansen trainsets with automated track inspection gear, and operate at full line speed, so they don't delay traffic.

[1] http://www.caltrain.com/projectsplans/Projects/Caltrain_Capi...

[2] https://en.wikipedia.org/wiki/Doctor_Yellow

Symbiote4• 7 years ago

Twice a year?!

It's not just Japan. France and Britain check their tracks every 2 weeks.

The British high speed measurement train has a site with pictures of some sensors, statistics ("captures around 10TB of image data every 440 miles") and a video showing more sensors, and an on-train server room.

https://www.networkrail.co.uk/running-the-railway/looking-af...

inferiorhuman13• 7 years ago

Well Caltrain doesn't have the wrong kind of snow to contend with.

Edit: In case the snow reference went over anyone's head: https://en.wikipedia.org/wiki/The_wrong_type_of_snow

goodcanadian• 7 years ago

From your link:

-The train surveys 4,800 miles of track every two weeks.

-The train covers 115,000 miles in a year.

It does not cover the entire system every two weeks.

EDIT: Wikipedia suggests a little over 10,000 miles of track in the UK, so it might be going over it all once a month though I suspect that heavily used high speed lines are covered more frequently than less used lines (which are more comparable to Caltrain).

dsfyu404ed• 7 years ago

Considering the other problems Caltrian has to deal with not spending much time checking rail straightness above and beyond what's required for the trains to operate seems like a very reasonable allocation of resources.

willyt• 7 years ago

I think the french do a better job though, the TGV is way smoother than Virgin Trains despite travelling at about twice the average speed. You could do a hand drawn sketch on a TGV, you can't do this on a UK train.

deathanatos• 7 years ago

Caltrain replaced all their track about 15 years ago, going to concrete ties and welded rail.

I live there. I took it to work for 6 years. (I stopped late last year.) The ties are mostly wood. There's some concrete ones in and around some stations, but the vast majority are wood.

Edit: https://www.youtube.com/watch?v=_s9HcflA_nY — there are more than I thought there were, but definitely not all of them.

m_eiman• 7 years ago

I've played with the thought of doing something like this to create a tool for assisting (and perhaps evaluating) drivers of buses:

* make a device that logs acceleration in various directions and performs suitable statistical analysis in real time. * combine it with passenger evaluations of ride behavior (tell users to press a button when the ride is uncomfortable) to train a neural net to evaluate ride smoothness * install devices in all buses, with a display placed in front of the driver that shows current ride comfort and warns if things get too problematic

rabryan35• 7 years ago

Fun read - great article for getting a better intuition on real world data analysis of dynamic systems (especially the part showing how bad conflating mean and median is).

Only gripe is it’s not an interactive notebook of some sort so others could play with the data.

jo-m• 7 years ago

Caltrain will soon get modern electrical trains [1] which have been in use for 8 years in Switzerland [2] and which will solve the vibration issues.

[1] https://calmod.org/electric-trains/

[2] https://en.wikipedia.org/wiki/Stadler_KISS

hcnews• 7 years ago

It seems like this isn't going to be a reality for another half-a-decade at least. (2+ years of constructions and 2+ years of productionization for consumer traffic).

GaryNumanVevo• 7 years ago

Unfortunately since there's only two tracks, one north, one south, it's kind of a farce. There isn't enough room for trains to over take.

smaslennikov• 7 years ago

I'm interested in the controls for this analysis, which I didn't notice in the article. Might've missed them.

I'm mainly talking about how the device was mounted - because if it's simply kept on person, a lot of this vibration is muffled, right?

Anechoic• 7 years ago

Soem comments on both the article, and other comments here:

- It's an interesting assessment, but without know the details of how he did the measurement, take care in interpreting the results. If he was using a smart-phone just lying on s seat of floor, the data are almost certainly (mostly) meaningless. If he was using a professional triaxial accelerometer that was rigidly or semi-rigidly attached to the floor, the data are more representative.

- With regards to comments about the data being related to track conditions. Passenger trains have two sets of suspension - the primary suspension between the axles and trucks (bogies for our European/Asian friends) and the secondary suspension between the truck and the passenger car. The secondary suspension is typically very soft for ride-quality purposes and is mostly isolated from the track. The vertical vibration levels measured here are almost entirely a result of the ride-quality decisions made by the manufacturer and agency engineers, and less so about the track condition.

- As pointed out by nhf and inferiorhuman, the side-to-side jerkiness (lateral vibfration) is likely related to track condition and is some combination of track alignment problems and wheel hunting (there the wheels on the solid axle bounce sideways back-and-forth between the parallel rails).

- Electrifying the line will have no affect on the cabin vibration unless Caltrains elects to purchase entirely new passenger cars with different secondary suspension design goals. If the new electric locomotives are pulling/pushing the same passenger coaches, the vibration will be the same.

- From personal experience I can tell you that the TransRapid TR08 (German system, used in China) can gave similar subjective levels of jerkiness when the guideway configuration has a sudden change.

laurencerowe3• 7 years ago

Caltrain is buying modern EMU stock where traction is distributed across the passenger cars.

hnews_account_1• 7 years ago

From the synopsis:

It's important to use dynamic measures in a dynamic system. I often see the mistake of using a static value for mean or kurtosis, unaware of how it changes over time.

This is both important to note, and unavoidable. You can account for changes in volatility in pure data analysis, but when developing mathematical models of time series, data stationarity has to be assumed for closed form solutions. Even if the volatility itself followed its own model (say garch could model it accurately), it can be iteratively reduced to a stationary series (I'm not sure if there's proof of this, but that's how most time series courses teach it - take first diff, second diff, log etc).

Markets are a similar level of complexity, but to compare anything to a reflexive system such as a market is kinda skirting the main issue of markets - reflexivity. You cannot find a closed form solution for it using current mathematics. Finance developed a distribution based mathematics called stochastic calculus (used in other fields, but it's the backbone of finance) to deal with this, but even there, the assumptions are highly restrictive compared to real world conditions.

Caltrain Vibration and Volatility Analysis

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