Could Bus Multiple Units (BMU)s bridge the bus-rail divide?

Here at Portland Transport, we (both editors and commenters) frequently like to engage in a bit of technical speculation, hoping for future improvements that will allow transit agencies to do more with less.  There’s lots of talk around here about electric buses, of driverless vehicles, of different vehicle configurations, and even more exotic concepts like Personal Rapid Transit (PRT) and bus/train hybrids.   And it’s a tradition ’round these parts to announce groundbreaking new transit technologies the day following March 31st.  :)

We also discuss the merits of bus vs rail a lot, and the various types thereof:  Local bus vs various grades of Bus Rapid Transit.  Streetcar vs light rail vs heavy rail (high-platform long-consist trains found in many large-city subway systems) vs commuter rail.  Some of these debates can get spirited.

Today, I’m going to discuss some utterly speculative technology that might help bridge the operational gap between large rubber-tired passenger-hauling vehicles running on paved roads (“bus”) and steel-wheeled vehicles running on steel rails.  Since I’m not aware of any existing, well-used name for the technology I’m about to discuss, I shall call it a Bus Multiple Unit (BMU).

More after the jump:

Advantages of each mode

Much has been written about bus vs rail, and we do not intend to rehash that here.  Things that are orthogonal to mode (amenities, frequency, exclusiveness of right-of-way) we will not discuss, nor will we consider any “culture” attributes of either bus or rail.   Some advantages of bus (specifically BRT) are given here; and they can be summarized into two broad categories:  a) Lower construction costs (almost none for local bus that uses an existing street network), and b) far greater operational flexibility (not tied to a fixed guideway). This operational flexbility allows things like phased construction, easier mixing of express and local services, branching topologies and “open BRT”, easier maintenance, and easier routing around incidents.

The main advantage of rail (unfortunately, I don’t seem to have a companion piece to the BRT piece listed above) are a) a few minor improvements in comfort, energy efficiency, and vehicle reliability owing to the physics of steel on steel, and b) the ability to couple railcars into trains.  The latter is by far the most important advantage of rail–if you want to haul twenty thousand persons per hour down a corridor of any length, buses won’t cut it.  Even if the drivers worked for free, the number of (independent) buses would overwhelm the system.  But when a single six-car subway train can hold close to a thousand passengers, and these can come every two minutes, this sort of passenger load is quite practical with rail.

In the Portland context, 20000 passengers per hour per direction is tremendous overkill, obviously; the main MAX trunk line does about a fifth of that.  Two-car MAX and WES trains and one-car Streetcars don’t take all that much advantage of this capability of rail.   But this discussion is not intended to be Portland-specific, and many systems worldwide do exploit rail’s capability to move large numbers of people.

So what is a BMU?

What, then,  is a BMU?  It’s essentially similar to a rail DMU (diesel multiple unit) or EMU (electric multiple unit), though with rubber tires and which runs on pavement.  (This discussion ignores the propulsion technology employed).  A multiple-unit (MU) railcar is a railcar which contains a full set of propulsion and braking systems, and can either run as an independent vehicle, or can be joined into trains.  In the latter configuration, its propulsion and braking systems are coupled together and placed under the control of a single operator.  WES uses DMUs (though some of the WES cars from Colorado Railcar lack engines); all MAX vehicles are EMUs.  The Streetcars are also capable of EMU operation, but are never so operated in the Portland Streetcar system.  The multiple unit railcar stands in contrast to locomotive-hauled trains, where one or more dedicated engine cars provide traction (more than a single locomotive is seldom necessary for passenger trains), and unpowered coaches carry  passengers (and numerous other types of unpowered cars can haul freight).

The BMU is essentially the same concept, applied to buses:  Two or more buses are coupled–possibly mechanically, possibly just electrically–into a train, and the operator of the lead bus drives them all in tandem.  Each bus in the consist has its own traction, steering (necessary for unguided operation), and braking, and could possibly be driven as an independent vehicle.  (Headless buses, intended only for coupling into trains but without an operator’s cab, are also possible).  This stands in contrast to how existing multiple-trailer trucks operate, which is similar to the locomotive-coach model:  A tractor provides traction, and it tows up to three (in Oregon) unpowered trailers.  (In some remote parts of the world, such as Western Australia, so-called “road trains” can get far longer than what is street-legal anywhere in the US).

Why bother?

Both locomotive-hauled railcars and “multiple unit” railcars (or single-car trams) are very old technology.  The fixed guideway provided by rails makes locomotive-powered configurations stable.  Engines can even push smaller consists safely (this is common on commuter rail lines, though certainly not for large freight trains). And the lack of a need to steer makes multiple-unit synchronization easy–only acceleration and breaking need to be coupled, which can happen more or less simultaneously.  The choice of locomotive-hauled vs MU-based trains in passenger operations is generally made based on the consist size–locomotives are more efficient when hauling large numbers of coaches, but are overkill for small consists.

But joining rubber-tired vehicles into trains is another matter.  The lack of a fixed guideway makes both single-powerplant and distributed-power trains difficult.  Single-powerplant configurations are commonly found in freight operations:  the tractor-trailer rig.  But tractor-trailers tend to be unstable and difficult to handle, with stability getting worse as more trailers are added.  Steering them is difficult, particularly in tight spaces, and the trailers tend to sway when being pulled at high speed.  Driving them in reverse also can be difficult.  Such configurations are generally only allowed for freight, and trains longer than three trailers are illegal in the US (and triples only legal on designated routes, and in a few states).

Articulated buses do exist, of course, but these are generally only powered in one section (sometimes the back section is powered, sometimes the middle set of wheels).  Bi-articulated buses (with three sections) are found in some countries, though usually only on dedicated rights-of-way; many other countries ban them.  (Even the longest bus in service that I’m aware of has less capacity than a single MAX car).A “multiple unit” configuration of buses or trucks would alleviate the stability problems associated with hauling trailers.   But until recently, such things have been technologically infeasible; even today, the relevant technology is in the research stage.

Current research

There are several different research programs which are of interest:

  • A prototype automated highway (a highway in which cars would be coupled in platoons and driven by computer control rather than humans, to permit much denser spacing of vehicles and less chance of accident) system was demonstrated prior to the turn of the century, in 1997.  This system, which required both modified highway infrastructure and modified vehicles, is an arguably more difficult problem than the BMU as described here–it needs to support arbitrary and dynamic configurations of vehicles (potentially different makes and models, owned by different people, which enter and leave the system at various and unpredictable times), rather than static consists of similar vehicles operated by the same agency.  Much of the research in automated highways has slowed down, as self-driving vehicles may be a better technological choice for passenger cars and taxis.
  • More recently, a European research program called Safe Road Trains for the Environment(SARTE) is studying the problem of platooning trucks, so the driver can safely navigate a fleet of trucks down a highway.  This project has had several successful trials, though at this point is not ready for production.  It is oriented towards freight applications, not transit, though the design goals of this project are similar to what might be necessary to build BMUs.
  • Some transit agencies actually operate manual bus platoons–buses that are scheduled to operate in tandem, but each one with its own driver. This isn’t commonly done–below a certain operating frequency you’re better off staggering bus runs to minimize headways–but in some cases, it can be useful, as it replaces uncontrolled and unpredictable bunching (which degrades service) with controlled bunching.
  • One other relevant technology is the guided busway–which means to allow buses to operate part of the time on the regular street network, and part of the time in a fixed guideway, in order to achieve (for part of the journey, at least) some of the advantages of rail.  The Adelaide O-Bahn is one example of this–buses entering the busway are mechanically coupled to a roadside track, and can operate the busway at high speed without need to steer.  This is mainly used to permit fast operation in a narrow right-of-way and to allow precise platform docking; this is not used to allow buses to be entrained.

How would this play in Portland?

Obviously, the purpose of all of this is to allow buses to a) retain their operational flexibility, at least somewhat, but b) haul larger number of passengers per operator, and be capable of handling large passenger loads, by coupling buses into trains for at least part of the journey.  It is unlikely that “bus trains” will be street-legal for operation on arbitrary public streets; even a two-bus platoon would be over 80′ long–about the length of a triple-trailer.  (Street-legal articulated buses are up to 67′ long).  So use of BMUs would be a BRT-focused operation.  But even within the context of a dedicated BRT line (such as the Southwest Corridor), a BMU platoon could offer the following advantages over light rail, while maintaining similar passenger capacity per driver.

  • Non-platooned buses could still use the busway, and it would still be far easier to support skip-stop or express service than with rail.
  • At certain stations, buses could leave the platoon and proceed to different destinations.  In the opposite direction, buses arriving at a transit center could assemble into a platoon, with one driver continuing to downtown, and the others waiting for a return platoon to split up.  This would pose quite a bit of logistical challenges, but might be preferable to riders than having to transfer from feeder buses onto a trunk line.
  • Busways are less susceptible to disruption due to incidents than are rail lines, as buses can steer around obstacles.  Even if an incident blocks a busway completely, it may be possible to certify parallel roadways for emergency bus-platoon operation, allowing platoons to navigate around closures of the primary route.
  • Bus platoons travelling on the mall could be treated like MAX trains, stopping at the MAX platforms.
  • During non-peak hours, the platoon size can be more easily reduced.  While a four-bus platoon (the largest you’ll probably get once inter-bus spacing is factored in; not sure five-bus platoons can safely be accommodated downtown) is appropriate for peak travel times, there’s no need to operate these at night.  With BMUs, especially if not physically coupled, the logistics of shrinking or growing the platoon size would likely be simpler than shrinking or growing a MAX train.  (Type 4 and 5 MAX vehicles come in pairs and cannot be run as singles; and Type 1 LRTs have to be coupled with Type 2 or Type 3 for ADA compliance).

BMU platoons probably wouldn’t make sense for the Powell/Division line, both because much more of this will likely be street-running rather than in a separate right-of-way, and the technology will probably not be ready in time for a desired opening by 2020 or so.


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