I won't repeat all the spin and numbers-twisting others exposed, I'll make some more general points instead.

First, on CO2 emissions associated with a light-rail (or any rail) line. The mayor sources are:

1. power generation for the vehicles,
2. manufacture of the vehicles (from mining the metal to transporting the finished product),
3. powering the track construction vehicles and machines,
4. the concrete in fixed infrastructures.

CO2 emission of the first and in part the third (e.g. tunnel boring machines, some drilling jumbos) depends on the share of generation modes behind the electricity used. It may be near zero. The per passenger-mile contribution of the last three, especially the last two, depends on depreciation time, e.g. how long the vehicle/built structure is in use. The calculation is not straightforward if we consider what will be only a sub-section of a longer line (as the Seattle tunnel will be).

Now, long depreciation times wouldn't be anything special for rail systems. For example, there is Vienna's system. Most lines are over a century old, and while Vienna also has the 100%-low-floor ULF type with its special single-wheeled system, still maybe half the well-maintained vehicles are of a type delivered from 1959.

No. 4731, an older E¹-type Vienna tram crosses the bridge over the Vienna creek where it reaches the Danube

It is also noteworthy that these anti-light-rail propagandists act as if experience with mass transit outside North America is irrelevant, ignoring that all the hundred or two European city councils deciding on keeping or building anew light rail must have had their reasons.

Unless the goal is to hold back public transport by advocating its least attractive and smallest-capacity mode, it is also mystifying why they consider buses and light rail as alternatives. In addition to the trams, Vienna also has a dense bus network, a subway network that is a mix of heavy and light metro, two overland tram lines, a rapid transit/commuter network on normal rails (S-Bahn) that also cris-crosses the city between the many main stations, and normal local trains.

The anti-rail types also advocated bikes. To make that a viable and safe alternative, of course they'd need infrastructure (preferably taken away from cars). Vienna has it, and more: Vienna's Citybike was the first modern rent-a-bike system (in 2003; Lyon's larger vélo'v system, much more noticed in the English-language media, followed two years later). It was created a year after a for-free bike rental system (Copenhagen model), which failed due to bike theft, a problem the new system solved with magnetic cards and user identification.

Citybike station Johannesgasse, on the corner of said street and the Parkring, one of the 54 stations of the system. As the empty stands indicate, lots of bikes were in use, in fact you can barely see a couple that just took away two at the street corner

Later on the streets in Frankfurt, I saw rent-a-bikes of a different system in use: ones rented by the German Railways. Next time there I'll use it.

Lower CO2 emissions aren't the only potential environmental benefit of light rail. There are noxious gases, and there is noise – imagine living in this street with bus traffic instead of the tram:

On line B, a new Citadis 403 (made by Alstom) travels along Strasbourg's Rue du Vieux Marché aux Vins with a silent swoosh in the early morning. The futuristic all-glass streamlined heads of the Eurotrams so much determined the image of the system and the city, that the newer 'colleagues' from the rival producer got similar heads

Strasbourg's traffic company, which also runs an extensive and expanding bus network, re-started its tram system in 1994, and achieved a reduction of downtown car traffic by 17% in the first few years (while elsewhere the general trend was upwards). Presently 39 km long with five lines (the last created a week after I was there) and wildly popular, it is still in expansion. This system is used as rapid transit, a fast core network for public transport in the city of 275,000. There is a tunnel section that includes the station under the main railway station, and future lines will go well out of the city (a tram-train, Karlsruhe system).

I had to ride in it myself.

Inside a Citadis 403 set on line A after boarding in station Porte de l'Hôpital. A rather spartan look inside, but comfortable

Shortly after, on the curve up the Pont Saint-Nicolas

I saw an even more dramatic example of how light rail can change a city after a TGV ride to Le Mans. With a budget of €290 million, the city of 145,000 builds a two-line network of 15 km. Construction work was nearly finished when I was there (opening is tabled for 17 November).

Le Mans, Boulevard René Levasseur: the tram tracks narrowed down a main road to a single-lane taxi road

The tramway's path was taken away from the cars – which in itself forced a dramatic change of transport modes already before opening. For now, that means more people walking & cycling, and that in much less noise and more attractive environment than in the next main street without tram.

The tramway is classless: it accesses both richer areas in the centre and the south-east, and poorer areas in the west of the city. Walking towards the west, it was most visible how the tramway changes its environment, too:

Looking back at Rue Gambetta from near its western end. Note that virtually all house fronts were re-painted, and most shops were under renovation or newly under installation. What was a grey street of depreciated homes (like Rue Fleury to the left) is becoming an amenable shopping & café street

While Le Mans and Strasbourg are small enough to have light rail as the core network of their public transport, in larger cities, they are better suited for secondary routes, especially orbital routes. Paris's new T1, T2 and T3 light rail lines serve such a purpose around the core city.

When crossing Paris on my mad rush, I saw work on the extension of the T2 line (which is to the west of the centre), and got a glimpse of what I think was a spur of the T1 line in the north, and the not yet one-year-old T4 'tram-train' nearby.

Frankfurt is a medium-size city, which had both tramways and S-Bahn normal rail rapid transit. Part of the former was moved underground to create a light metro system, but surface lines remained, too. When I lived in/near Frankfurt, almost two decades ago, old post-war vehicles and seventies stock in less attractive orange/beige rolled over them. Now all seem to have been replaced with new low-floor trams in turquoise.

New type S tram No. 223 (a Bombardier Flexity) in front of the European Central Bank headquarters. Even the upper-class can travel on public transport. But the advertisement of a car maintenance company is not the best combination...

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