Trams are a seemingly expensive outlay for a city. They require permanent tracks and overhead wires. At least that’s how the figures can often initially appear. But there is more to it than basic installation cost. What of the energy required to operate, maintain and recycle our transport network? Once installed trams are one of the lowest energy and most sustainable mass transit systems a city can operate. Much lighter than trains, without the need for often cost prohibitive and energy-intensive underground tunnels that a metro system requires. Trams run on hard wheels and rails that can be fully recycled and have much lower rolling resistance than soft rubber tyres. They are plugged directly into the mains, negating the need for energy and resource intensive batteries that need their own separate and often more expensive charging infrastructure.
Below is a breakdown comparing trams and buses. Because the length of trams can vary, a single carriage, which is also comparable with the weight of a double-decker bus, has been used in these calculations. In reality, a three-car tram can carry as many as 140 passengers, standing and seating, compared with the new electric London buses, which propose to have a capacity of 90.
The main characteristics affecting energy consumption are rolling resistance, drive efficiency, drag coefficient and frontal area and weight.
The rolling resistance of a hard, almost inflexible tram wheel, on a similarly inflexible track has a coefficient of rolling resistance of approximately 0.001, approximately ten times lower than a bus tyre (0.01) and as much as twenty times lower than a correctly inflated car tyre on asphalt (0.02).
A double decker bus travelling at 30mph along a smooth tarmac road would need 24.9 kW to keep it moving. The same bus running on tram wheels on metal tracks would need only 3.5 kW. More than 7 times the power.
