Summary
Vibration Analysis Revisited: Four years ago, I conducted a vibration analysis on the old diesel Caltrain cars. Recently, I revisited this analysis with the new electric Caltrain to compare ride volatility between the two.
Substantial Ride Quality Improvements: The transition from diesel to electric trains has led to substantial improvements in ride quality across all axes of movement. The electric trains demonstrate significantly lower volatility and fewer extreme events, contributing to a much smoother, safer, and more comfortable ride for passengers. These improvements are a testament to the effectiveness of the technological upgrades made in the shift to electric propulsion.
70% Smoother Ride: The electric Caltrain offers an impressive 70% smoother ride compared to its diesel predecessor.
Enhanced Passenger Comfort: The most notable reductions in volatility occur in the front-back (X-axis) and side-to-side (Y-axis) movements. This leads to a more stable and less jarring ride, especially during acceleration, deceleration, and lateral movements, significantly enhancing passenger comfort.
Premise/Motivation
Given my previous vibration analysis of the legacy diesel Caltrain cars, I saw a valuable opportunity to apply the same data collection and analysis methods to the new, quieter electric trains. The key question I aimed to answer was: just how much smoother is the new electric Caltrain ride compared to the diesel version?
Even without analyzing the data, the difference in ride quality is apparent—the electric trains provide a noticeably smoother experience, as any rider can feel. However, the purpose of this analysis is to quantify that smoothness. Let’s dive into the data to see just how significant the improvement is…
Focus on Side-to-Side Movement:
Side-to-side movement (Y-axis) is often the most variable due to track imperfections, curves, and other factors, making it a major contributor to overall ride volatility. In my previous analysis of the diesel trains, side-to-side volatility was the primary source of discomfort. Therefore, any reduction in Y-axis volatility on the new electric trains could significantly enhance passenger comfort.
Reiteration of methodology
For this analysis, I used a three-dimensional accelerometer to measure vibrations along the front-to-back (X), side-to-side (Y), and up-and-down (Z) axes. The accelerometer sampled acceleration data 250 times per second. I captured approximately 25 minutes of data during a ride on a bullet train in August 2024. For this comparison, I focused on the longest uninterrupted segment of the ride, about 15 minutes (~225,000 data points), where the train reached maximum speed.
The analysis examines volatility in the X, Y, and Z directions, identifying jolts and jerks, and compares the measured volatility between the old diesel trains and the new electric trains. Key metrics include frequency distributions, median absolute deviation, and kurtosis.
Rider Experience of the new Electric Train
For the up-down direction, there is one large observation from the electric train, but generally speaking, of the 5 largest impulses, 3 are from the diesel train, and 2 from electric.
Axis-Specific Improvements in Ride Smoothness:
Median absolute deviation provides a robust measure of the typical volatility or fluctuation in acceleration. Higher values indicate more frequent and significant changes, which can correspond to a rougher ride.
(X = Front-Back, Y = Side-to-Side, Z = Up-Down)
Diesel Train:
X Axis: 0.088
Y Axis: 0.113
Z Axis: 0.085
Electric Train:
X Axis: 0.024
Y Axis: 0.032
Z Axis: 0.03
Analysis:
The median absolute deviation has decreased dramatically across all axes, further supporting the conclusion that the electric train provides a significantly smoother ride. The reductions are as follows:
X Axis: Reduced by about 73%.
Y Axis: Reduced by about 72%.
Z Axis: Reduced by about 65%.
These reductions indicate that the typical deviations from the median are much smaller in the electric train, leading to fewer and less intense movements, whether lateral, vertical, or longitudinal.
Side-by-Side Volatility Comparison
Notice the magnitude and frequency difference of Side-to-Side jolts/jerks in the diesel vs. electric train
The electric train's front-back jerks are not even noticeable compared to diesel!
Kurtosis:
Kurtosis provides insight into the "tailedness" of the distribution. Higher kurtosis indicates more frequent extreme values, suggesting more outliers or sudden, sharp movements.
Diesel Train:
X Axis: 1.36
Y Axis: 1.62
Z Axis: 3.92
Electric Train:
X Axis: 1.08
Y Axis: 1.64
Z Axis: 3.78
Analysis:
The kurtosis for the X axis (Front-Back) has decreased from 1.36 to 1.08, suggesting that the distribution of acceleration in the electric train has become more platykurtic, with fewer extreme outliers. This indicates that the electric train experiences fewer extreme jolts in the front-back direction, making the ride smoother during acceleration and braking.
The Y axis (Side-to-Side) kurtosis remains relatively stable, indicating that while the average and typical variability have decreased (as shown by the MAD), the occurrence of extreme lateral movements hasn't changed much. This suggests that while the ride is generally smoother, the electric train may still experience occasional sharp lateral jolts, possibly due to track conditions or specific maneuvers.
The Z axis (Up-Down) kurtosis has decreased slightly from 3.92 to 3.78, indicating a small reduction in extreme vertical jolts. However, the kurtosis remains high, reflecting that significant vertical impacts still occur, although they might be less frequent or severe compared to the diesel train.
Overall Interpretation and Conclusion
Substantial Improvement in Ride Quality:
The electric train shows a marked improvement in reducing both the average and typical volatility across all axes. The reductions in both mean and median absolute deviation indicate a much smoother ride with fewer and less severe fluctuations.
Reduced Extreme Events (Outliers):
The decrease in kurtosis for the X and Z axes suggests fewer extreme jolts, particularly in the front-back direction, contributing to a more stable and predictable ride. However, the Y axis kurtosis remaining stable indicates that while the ride is generally smoother, some sharp lateral movements may still occur.
Technological Advancements:
These improvements can be attributed to advancements in train technology, such as better weight distribution, more effective suspension systems, and smoother power delivery. The shift to electric propulsion likely plays a significant role in these improvements, as electric trains typically offer more consistent and controlled power management.
Passenger Comfort and Safety:
The significant reductions in volatility, particularly in the X and Y axes, translate directly to enhanced passenger comfort. The electric trains provide a more stable and less jarring experience, reducing the likelihood of discomfort or motion sickness, especially during acceleration, deceleration, and lateral movements.
Thanks to all those who worked on the Caltrain Electrification upgrades over the years!
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