The Hybrid DMU, Unicorn of the Rails

The hybrid DMU is a diesel multiple-unit train with a twist: it has on-board energy storage to enable energy recovery, similar to an electric train that feeds power back into the grid when slowing down.  Just as in a hybrid automobile, this energy store helps to start the train rolling again, and reduces energy consumption in stop-and-go operations.  Peninsula cities and stakeholders, ever more astute on rail matters, perceive three important benefits in the hybrid DMU:

1. It keeps high-speed rail out.  In discussions of Caltrain electrification and the slow beating-around-the-bush process that is leading up to the certification of the electrification EIR, high-speed rail has become the central issue. Electrification is viewed in some quarters as the camel's nose under the tent, so a renewed push is underway to ensure that all the alternatives other than electrification have been duly considered.
    
2. It keeps unsightly high-voltage poles and wiring at bay, preserving views and presumably residential property values in some of the most affluent areas in the nation.
    
3. It spares Caltrain, the perennially funding-starved agency, the burden of spending $785 million of scarce capital dollars to string wires over its tracks.

The hybrid DMU is viewed as a synergistic technology that solves all three issues in one fell swoop, in a classic Silicon Valley win-win-win.

There's only one little problem: it doesn't really exist.

Okay, almost.  There do exist a handful of hybrid DMUs in Japan.  These advanced technology trains, instantly knowable to any city staffer via a simple Google search (keyword hint: 'hybrid DMU'), operate in Japan.  The fleet numbers three cars on one line, and ten cars spread among four other lines.  Each car seats about 45 and tops out at 60 mph.  Their power output is less than a Chevy Tahoe hybrid's.

The Law of Diesel Trains

To understand why the hybrid DMU will never work for the peninsula corridor, look no further than the laws of physics.  What Caltrain needs is a singular focus on better service: quicker trips, more frequent stops, higher capacity, and less waiting for the next train.  That requires big, fast trains with one key quality: punchy acceleration, precisely the reason why the EMU (electric multiple unit) powered by high-voltage overhead lines was invented and perfected.  To achieve high acceleration, the laws of physics dictate high power and low weight.

A diesel train makes all its power on board and sends it to electric motors that drive the wheels; it is essentially a rolling mini power plant.  The nicest and newest rolling mini power plants used by Caltrain today generate 3,600 horsepower for a million-pound train.  In metric units, that's about 6 kW/ton, and as any Caltrain rider can attest, it doesn't exactly pin you to your seat.  The problem is that rolling mini power plants are heavy, and if you want more power, you'll need to haul around even more weight.  This is the Law of Diesel Trains, directly derived from Newton's Laws of Motion.  More weight does little to help with acceleration, so diesel basically can't scale up.

The EMU, on the other hand, doesn't schlep around a rolling mini power plant.  Its electricity is generated off-board by a real power plant, of the PG&E variety.  The electric grid, hooked up to gigawatts of generating capacity, can provide essentially limitless power to a train.  That endows a typical EMU (of the sort available off-the-shelf from many manufacturers) with a power-to-weight ratio of about 12 kW/ton, or double the giddy-up of a diesel train.  For short bursts of acceleration, high-voltage EMUs can briefly exceed their continuous power rating and draw even more power, hitting up to 18 kW/ton.  For those still keeping track, that's triple the acceleration of Caltrain.  For reference, BART cars achieve a respectable 15 kW/ton.

So what do the Japanese know about hybrid DMUs that we don't?  Most importantly, they do not claim zippy acceleration as a benefit.  The hybrid DMU does three things for them: save about 10% on fuel, cut down nitrous oxide emissions, and cut the noise of an idling train down to an electric whisper. Beyond those benefits, the 'D' in DMU makes it follow the Law of Diesel Trains.  The Japanese hybrid DMUs manage barely 5 kW/ton, probably because they haul around not just a mini power plant, but also big heavy batteries.  Here in the US, more stringent crashworthiness standards would make such trains even heavier and their performance even more anemic.

The inescapable conclusions are thus:

• Hybrid DMUs provide only about one third of the acceleration required to enable meaningful Caltrain service improvements. A simple technical litmus test for future Caltrain rolling stock is the power-to-weight ratio, required to be at least 12 to 15 kW/ton.  Hybrid DMUs don't qualify.
• Hybrid DMU technology has never been scaled up beyond the size of a bus.
• Hybrid DMU technology inherently cannot be scaled up to achieve higher power-to-weight and acceleration in large (600 - 1000 passenger) configurations.

The mystical powers ascribed to hybrid DMUs by peninsula stakeholders rightfully earns them the nickname of 'Unicorn of the Rails.'  It's time for them to realize that by undermining the choice of EMU technology and promoting hybrid DMUs, they are also undermining the future of Caltrain--intentionally or not.  Caltrain can be faulted for many things, but their choice of high-voltage EMU technology as the path to modernization is unequivocally correct and technically justified, regardless of what happens with high-speed rail.

Calling All Service Planners

The recent talk of phased implementation and a "blended" Caltrain + HSR system has some people proposing new service patterns and new timetables. That's a healthy thing: service planning should always drive infrastructure decisions. To ground this discussion in reality, these proposed service patterns must reflect realistic train performance that doesn't require Star Trek warp drives (or, for that matter, four tracks everywhere from San Francisco to San Jose...)

Using a Train Performance Calculator, we can find out how long any given train will take to travel from point A to point B, taking into account grades, rail adhesion, aerodynamic drag, traction and braking curves, line speed limits, etc. The results of such calculations are presented below for four key types of rolling stock on the peninsula rail corridor. With these run times, you've got all the building blocks you need to build your own strings, and from those strings, your own timetable.

The trip times can be downloaded as an Excel spreadsheet (82 kB) or a PDF document (106 kB) with eight separate tables (each in its separate worksheet) corresponding to the scenarios described below. They are reasonably accurate, but perhaps not down to the second.

Caltrain Diesel Train

The prototype for the first set of run times is a standard Caltrain consist, with one F40 locomotive and five gallery cars. The diesel locomotive is rated at 3200 hp, and the entire train weighs 420 metric tons fully loaded with 500 passengers. The train is technically capable of reaching a top speed of 100 mph, although signal system restrictions (planned to be removed) constrain it to 79 mph today.

Note: despite their bullet nose, the Baby Bullet trains have essentially the same performance.

Diesel Multiple Unit (DMU)

The prototype for the following run times is a Siemens Desiro Classic DMU. This DMU is in common use around the world, including here in the United States (although it is not compliant with FRA crash regulations). The train performance specs are based on San Diego's Sprinter, with a four-car consist as shown in the photo. Total power output is 1680 hp total for a train weighing 392,000 lb fully loaded. Top speed is 75 mph; because of this limitation, the run times are valid regardless of the track speed limit.

Electric Multiple Unit (EMU)

The next set of run times is for a Stadler KISS EMU. This six-car double-deck EMU, similar to the types under consideration for Caltrain's electrification project, has a top speed of 125 mph. The spec sheet shows that the train weighs about 325 metric tons fully loaded with 500 passengers, and is rated at 4000 kW (5300 horsepower).

The EMU's secret weapon is the ability to unleash a short-term (few minutes) burst of 6000 kW (over 8000 horsepower), which takes it into the same performance league as a high-speed train. This is handy for performing overtakes on the express tracks without disrupting high-speed traffic--a key capability for a "blended" Caltrain + HSR plan. This trick is not possible with a DMU, which is more akin to a moped entering a freeway. The run times below are for the same train using its 6000 kW short-term rating, to be used sparingly.

High-Speed Train

The final set of run times is for a state-of-the-art high-speed train of the sort that might someday be used in California. It is an 11-car Alstom AGV with a top speed of 220 mph, but used in this case at far lower speeds. The train weighs 404 metric tons and has a very high power output of 9120 kW (over 12,000 hp) as is common for high-speed trains. Generic high-speed train specifications have been compiled by the CHSRA.

If you missed the download link above, here it is again for all the above scenarios: Excel spreadsheet (82 kB) or PDF document (106 kB)

Rules of Thumb

1. These run times are start-to-stop times only, with no intermediate stops, and do not include dwell or padding. Think of them as the fastest possible timing from Point A to Point B without stopping.
   
   
2. Dwell time at stations is not included, and must be added separately. Caltrain dwell times can generally be assumed to be about 45 seconds if level boarding is not provided (i.e. there are steps into the train), or 30 seconds if level boarding is provided. Reduced dwell times can provide enormous savings for frequent-stop commuter trains. High-speed train dwell times should be (per TM-4.2 Phase I Service Plan) 90 seconds at intermediate peninsula stops, and 120 seconds in San Jose.
   
   
3. Padding is not included, and must be added separately. Without padding, a timetable can only be run under perfect conditions. In the real world, stuff happens, and padding ensures that the entire timetable doesn't collapse like a row of dominoes. A good rule of thumb is 20 seconds of padding per stop.
   
   
4. Speed limits ought to be selected carefully. It is unlikely that speed limits will increase where grade crossings are still present. (While this is technically permissible under FRA regulations, state regulations are more restrictive, based on the risk profile of each individual crossing. On the peninsula these crossings typically have a lot of road traffic.)

Building Strings for a Timetable

With the preceding rules of thumb in mind, it becomes a reasonably straightforward exercise to build a "string" that describes the position versus time of any given train, whether it be local, limited, express or long-distance HSR--based on the prevailing speed limit, train type, and stopping pattern. For example, we can construct the timetable for Caltrain 216, departing 4th & King at 7:19 AM, using the following building blocks:

• 4th & King to San Bruno: 691 seconds
• Station dwell at San Bruno + padding: 45 + 20 = 65 seconds
• San Bruno to Burlingame: 314 seconds
• Station dwell at Burlingame + padding = 65 seconds
• Burlingame to San Mateo: 148 seconds
• etc.

By the time you get to San Jose, it all adds up to an 8:25 AM arrival... three minutes early by Caltrain's timetable, but that has some extra generous padding at the end of the run, in order to juice their on-time statistics.

Once you've built a few "master" strings for the basic Caltrain and HSR service patterns that you envision, you can put them on a spreadsheet and slide them around to build the best-possible clockface timetable. When you do this, make sure that no two strings in the same direction of travel ever come within less than about 3 minutes of each other--otherwise, passing tracks will have to be added to allow the strings to touch or cross. This process illustrates how a timetable can tell you where the four-track sections are actually needed.

Bear in mind the limitations of this simplified approach. The most beautiful timetable can fall apart when things don't go according to plan. The pros use expensive software that can figure out how robust a particular timetable will be to the inevitable perturbations, something that factors heavily into service planning. That particular aspect of the problem isn't dealt with here.

Happy timetable building.

The Small Print

The trip times were calculated in Octave using numerical integration of the differential equations of motion. Traction, friction and drag curves are taken from train specification sheets; where not available, these are calculated based on weight on drivers and power (for traction) and the modified Davis equation (for friction and drag). Curve and terminal area speed restrictions are included. The speed limit assumptions include: 35 mph in the Transbay Transit Center; 40 mph out to 4th & King; 65 mph at Bayshore; 70 mph at Sierra Point; 75 mph at San Bruno (assumes new grade separation); 75 mph at Millbrae; 85 mph at Hayward Park; 80 mph at Palo Alto; 70 mph at Lawrence / Bowers; 45 mph in the San Jose approach. All trip times take into account the 0.6 mile discontinuity in the milepost numbering near CP Coast. All trip times assume that the train accelerates and brakes at the maximum service rate, and maintains a margin of 2 mph below the speed limit at all times. Small (few-second) differences in northbound vs. southbound trip times are ignored. Results should be accurate to about ten seconds. Your Mileage May Vary.

The Future of Caltrain, Without HSR

Today, I had the opportunity to participate in the Save Our Caltrain summit, organized by the grassroots group Friends of Caltrain. It was a pleasure to meet many blog readers in person. My presentation focused on how capital investment in Caltrain should be carefully targeted to increase service quality, grow ridership, and put Caltrain on a sustainable path that breaks out of the debilitating spiral of cost-cutting. Some participants at the summit asked for my presentation slides, so here they are:

Download Slides (1.1 MB PDF file)

There wasn't enough time in six minutes to drive home some very important points.

• In deciding which improvements to make, it is of the utmost importance for all stakeholders to agree on the metrics that will be used to determine the relative merits of various proposals. Until agreement is established on the scoring framework, debating the particular merits of this or that project is an arbitrary, subjective exercise with no reasonable chance of closure. You can read more about what I think are the metrics that matter.
  
• Caltrain has failed miserably in marketing the electrification project to the public. It is often sold on the basis of lower emissions, "lower" operating cost, lower noise, lower fuel costs, greater comfort, etc. The bottom line is that electrification is about trip times. Caltrain's product is its timetable, and improving the timetable should be the organization's singular focus. The potential support that peninsula communities could provide to the project is not being tapped because Caltrain has failed to communicate the tangible benefits of electrification to each individual city. Caltrain must promise and deliver a future timetable, and identify specifically how each city would be better served. (How many more trains stopping per hour, and trip times to key destinations.)
  
• There are several reasons beyond poor marketing that explain why the Caltrain electrification project has been in the works forever, and threatens to continue to languish in that perpetual state of indecision--despite its very high return on investment. None of the reasons are technical. It's all politics.
  
  1. Electrification will not create many local jobs because it is a systems contract that contains primarily highly manufactured goods.
  2. Electrification does not involve the pouring of enormous amounts of concrete, so the local transportation-industrial complex (aligned around large civil engineering and construction firms) is not inclined to support it.
  3. Electrification would undermine the justification for BART on the peninsula. The federal funding for new transit mega-projects is scored in part on the basis of cost per new rider, and allowing Caltrain to tap those new riders would make BART look far less attractive an investment.
  4. Electrification is considered by many anti-HSR activists to be the camel's nose under the tent. It potentially hastens the day when high-speed trains will reach San Francisco. The mere threat of litigation has put the project on the back burner since last April.
  All these political obstacles must be overcome.
  
• DMUs (Diesel Multiple Units) were often mentioned at the summit as an alternative to electrification. This solution will not achieve the required time savings! Electric trains have a much higher power-to-weight ratio that gives them the required acceleration. It cannot be emphasized enough that the quality desired of Caltrain's new fleet is acceleration. Top speed matters much less. When picking a new train fleet, the key performance spec is power-to-weight ratio... today's diesels are at about 6 kW/metric ton, and we need to be at least in the 12-15 kW/ton range. A few people mentioned Japanese hybrid DMUs (diesel with battery storage), but those extremely lightweight DMUs would never meet the crash standards that are being required of Caltrain. If these hybrid trains were imported here, their structural mass would have to be increased and their acceleration would be insufficient to provide much of a benefit. If it can't do 12 kW/ton, forget about it.
  
• The mid-line overtake isn't half bad for offering continuing service to San Francisco Transbay, should the high-speed rail project initially terminate in San Jose.
  
• Some people asked about the doctored BART EMU photo. It is based on a Stadler KISS EMU (an off-the-shelf European design) that would be perfect for the peninsula. Here is the big version of the rendering.

There was no time for questions, so please post any queries in the comment section below.