Recently transport consultant Jarrett Walker posted an interesting article on his Human Transit blog regarding the intrinsic and non-intrinsic differences between rail and bus technology, drawing on an article in the Infrastructurist blog which had asked its readers if streetcars (trams) were better than buses and if so, why.

This article identified 36 reasons for the superiority of trams. These Jarrett classified into three groups:

• “misdirected differences”, for example, those such as electric propulsion and dedicated rights of way which are often associated with rail-based technology but which can be (and have been) applied to buses;
• “cultural feedback effects” which relate to the way trains, trams and buses are treated culturally, for example perceptions that “buses are for poor people” or that rail-based systems are more permanent are not technical differences but rather cultural constructs that can become self-reinforcing;
• “intrinsic differences”. Those are the “real” technical differences between bus and rail such as capacity, ride quality, energy efficiency and costs. Jarrett claimed that there were only seven such differences and only three were clearly to rail’s advantage.

This is a thoughtful piece and provoked lively debate. I didn’t entirely agree with Jarrett’s approach and took a slightly different tack. This is an edited version of my response (it’s useful but not essential to read Jarrett’s piece first).

 I think the approach of trying to identify the real “uniqueness” of bus and rail technology is important but I don’t think can be completely divorced from the nature of the corridors and services involved.

This is especially true at the “extremes”. For example, to provide a small, localised transport service in low density outer-suburban or rural areas (either on demand or on a scheduled basis), bus technology using existing roads is clearly the only answer because of its low cost and flexibility. Nobody would contemplate using any form of rail technology for something like this.

Translohr guided vehicle in Clermont-Ferrand, France (all images from Wikipedia)

At the other end of the scale would be high speed and/or high frequency and high capacity services in dedicated and completely grade-separated corridors such as high-speed suburban and interurban rail services or underground metros. Even if all the “non-intrinsic” differences were eliminated, bus technology would simply not be able to offer the same level of service in these situations for a number of reasons.

An example is the Perth-Mandurah high-speed suburban rail line in WA, where buses would be unlikely to be able to offer the same fast travel times even if they had a dedicated corridor (in this context, maximum speed is probably another intrinsic difference between the two technologies, though it has to be acknowledged that for many transport corridors this probably isn’t a significant issue).

Adelaide O-Bahn (guided busway)

In the case of metros, if you were to take buses as your starting point, eliminated the non-intrinsic differences and minimised the intrinsic ones in order to meet the demands of providing a high-capacity, underground service, you would have to add electric traction, vehicle guidance, automated signalling systems, multiple trailers, platform loading etc, etc. The result would then be virtually indistinguishable from rail technology, apart from running on rubber tyres, so it would end up looking pretty much like the tyre-based Paris metro lines – and probably cost about the same as metal rail technology anyway.

In fact the Infrastructurist article (that inspired Jarrett’s article) talks mainly about the differences between streetcars and buses, not rail versus road-based transport generally. The points Jarrett has raised are also most relevant when applied to street-running bus services and trams, or a little further up the foodchain, busways and dedicated light rail corridors. In both cases there is much more overlap between bus and rail-based services than at the extremes I mentioned earlier.

Melbourne tram leased from Mulhouse

There are a few other issues worth noting regarding this debate, however. Generally speaking, bus technology has to be more fully “optioned-up” from its base form than rail to eliminate the non-intrinsic factors and mitigate the others. Ride quality is a good example. Basic trams running on well-maintained tracks in mixed-flow situations will usually offer a better ride than buses in a similar situation.

It is only in dedicated corridors that the latter have the potential to approach the standard of the former and even this requires careful planning and additional construction costs – for example, the decision to build the Western Sydney bus transitways to light rail standard to allow for their potential conversion, or the extent to which the Brisbane busways were engineered to improve ride quality.

Los Angeles light rail

In some respects it’s a bit like choosing between a top-of-the-range car with all the extras built in or the basic model and then adding on the extras. Often the latter would end up being more expensive and the technology not as well integrated as the “de-luxe” model. In the case of busways, I understand that the cost of the dedicated busways in Sydney and Brisbane approached that of light rail.

This isn’t necessarily a reason not to build busways if they offer other advantages, but if all the options Jarrett’s article mentioned are added (such as electric traction, vehicle guidance, etc) to mitigate the “non-intrinsic” differences, they could well cost more than light rail. This expense also often results in trade-offs being made in relation to busways, as highlighted in many of the responses to Jarrett’s post.

A couple of other observations. I would add another intrinsic difference – rail, by its nature, has a built-in standardised “guidance system”. While there are differences in rail gauges and other associated technology, and railcar or tram of a given gauge will conform to this basic guidance system. This means it is possible for many tram and light rail systems to purchase vehicles off the shelf or lease them from other systems with little modification.

Cologne light rail operating as a metro

Of course, basic buses used for ordinary street-running of course have very few compatibility issues, but the situation is much more complex in relation to guided buses. There are at least four different basic technologies (kerb-guided, central rail, optical and electromagnetic) in use and eight or nine incompatible guidance systems based on these technologies. This isn’t to argue against the concept of guided busways, which have a lot of potential, but unless a dominant guidance technology emerges, rail should be considered to have an advantage in this area. You simply can’t buy a guided bus off the shelf.

Another interesting area of difference is flexibility and scale. Of course, buses have an advantage that the same vehicles can go very easily from running mixed-flow systems servicing local bus-stops in outer suburban areas to providing dedicated busway services in higher-density corridors.

While trams and light rail can’t match the flexibility of buses at the suburban level, they can be more easily scaled up at the other end. The transport system in Cologne and some other German cities are good examples, where trams can start in outer suburbs with basic on-street services, then run in dedicated corridors as light-rail services in middle-ring suburbs, much like busways – however, they then go underground to provide metro-like high capacity services in the CBD, a feat that would be much more difficult to do with buses.