The choice of infrastructure is one of the most frequent debates about public transport: whether to build a rail-based line (heavy rail, light rail and anything in between) or expand a bus service with costs and benefits of each option. The best possible answer, in short, is: it all depends on the particular circumstances, mainly the goals, major constraints (“What can go wrong and why?”) and the budget.

The following analysis compares 6 modes of transport across 6 parameters and it is intended to help when making a decision about the development of a transport network. It assumes a perspective of an average European mid- to large-size city that already has some but not all of the modes listed and likewise it some infrastructure, most likely fragmented. (I am concentrating on high capacity modes of transport that are at least to some extent independent from other traffic.)

The conclusions are summarised in a table below:

Individual parameters:

1. Costs of new infrastructure:

• Tram or light rail (an additional line) – medium: new infrastructure (line extension or a link) added to the existing one
• BRT – medium: new infrastructure (a street with dedicated lanes) added to the existing one
• Commuter rail – none: maximising the use of existing infrastructure by modifying train routes on existing lines and adjusting the frequency *
• Tram-train – small: merging the existing infrastructure into a single network by building links between train and tram lines
• Premetro – high: new infrastructure (a city centre tram tunnel) added to the existing one
• Metro – very high: new infrastructure independent of the existing one

2. Costs of new vehicles:

• Tram or light rail (an additional line) – medium: using existing fleet with some purchases (to maintain the frequency on the existing network)
• BRT – medium: a completely new fleet needed, in order achieve a higher capacity than regular buses
• Commuter rail – none: maximising the use of existing fleet *
• Tram-train – high: a completely new fleet needed
• Premetro – medium: using existing fleet with some purchases (to avail of a higher network capacity)
• Metro – high: a completely new fleet needed

3. Degree of separation from other traffic:

• Tram or light rail (an additional line) – medium: tracks partially on street, partially separated (mainly outside of the city centre)
• Bus Rapid Transit (BRT) – large where a dedicated lane exists with a larger degree of separation than a bus lane
• Commuter rail – full: tracks at a rail line standard (limited number of level crossings outside of the city centre)
• Tram-train – large: fully separated on railway lines, medium on tram lines
• Premetro – large: fully separated tracks in a city centre tunnel, medium on tram lines outside of the city centre
• Metro – full: tracks at a rail line standard (no level crossings)

4. Route flexibility:

• Tram or light rail (an additional line) – large, assuming a sizeable network already exists
• BRT – small due to a dilemma of high cost (many streets with dedicated lanes) or a significantly reduced attractiveness (if routed through mixed traffic) **
• Commuter rail – large, assuming a sizeable network already exists
• Tram-train – very large, assuming both tram and rail networks already exist
• Premetro – large, assuming a sizeable network already exists
• Metro – small: each line has a separate tunnel

5. Line capacity (typically in line with vehicle capacity):

• Tram or light rail (an additional line) – medium: larger on separated sections, smaller outside of them (g. in city centres)
• BRT – medium: larger on dedicated lanes, smaller outside of them
• Commuter rail – very large: following mainline rail standards
• Tram-train – large: higher on rail tracks, smaller on tram tracks
• Premetro – large: higher in a city centre tunnel, smaller outside of it
• Metro – very large: same as mainline rail

6. Interchange requirement:

• Tram or light rail (an additional line) – small: many direct routes (assuming a sizeable network exists)
• BRT – large: a few direct routes (otherwise a high cost of dedicated lanes on many streets)
• Commuter rail – large: lines frequently bypassing the core city centre or large residential areas
• Tram-train – small: direct routes from suburban areas to the city centre
• Premetro – small: many direct routes
• Metro – large: each line has typically only one possible route

* The descriptions above refer to a start of commuter rail operations. Serving additional areas and increasing frequency requires large costs related to e.g. building a new line or adding additional tracks in the city centre (in order to avoid track sharing with long-distance trains).

** A speed comparable with rail-based transport requires dedicated lanes on the entire route. A compromise of partial routing outside of dedicated lanes (e.g. in the city centre) creates a risk of a noticeable drop in speed and BRT buses getting caught in traffic jams.