[mikepavone]

> I think the answer is that they are still building with the same weight as a train, rather than a bus.

A non-trivial part of this difference is that train cars are generally bigger than a bus. Light rail is generally more bus sized and they are generally closer in weight (though still heavier).

> * Homogenous cars. They all have traction motors

Electric passenger rail systems generally already use EMUs which have a power unit per-car or per pair of cars.

> small batteries

I'm not sure how you're going to have a small battery in a bus-sized vehicle that needs to operate fairly continuously for a good portion of the day unless this is on a partially electrified ROW. EMUs with smaller batteries to serve such routes already exist FWIW.

I think there's a reasonable case to be made to adjust US passenger rail regulations to allow lighter cars (especially in the context of high-speed rail), but allowing pneumatic tires seems like a poor motivation for it.

[inferiorhuman]

  A non-trivial part of this difference is that train cars are generally bigger
  than a bus. Light rail is generally more bus sized and they are generally
  closer in weight (though still heavier).

Muni's articulated streetcars weigh about 100,000 lbs each while typically the max weight for a laden tractor trailer is around 80,000 lbs. Light rail is a marketing term, not indicative of the actual weight.

[mikepavone]

Not sure where you're getting the 100k lbs figure from. The Siemens S200 LRVs are supposedly 76k lbs as deployed in SF. Traditional "heavy" rail EMUs often exceed 100k pounds per car (Kawasaki M8s used by Metro North range from 97k to 144k lbs). The unarticulated New Flyer XT40 trolley buses that Muni also uses are only about 32k lbs by comparison, but they're only 40 feet long whereas the S200 LRV is 75 feet long. The articulated XT60s (60 feet long) would be a better point of comparison, but I can't find a weight for those.

[inferiorhuman]

SFMTA is saying 80,000lbs for the LRV2/3 and 78,000lbs for the LRV4 – I rounded it up to 100,000 because the last time I thought about their weight was when they were new (and their weight as a point of ire). Breda over promised and underdelivered and the LRV2s were quite a bit heavier than expected prompting a lot of teeth gnashing (and eventually a lawsuit if memory serves) from residents. Wikipedia pegs the New Flyers at about 45,000 lbs. BART's cars were in the 50,000 lb range due to their extensive use of aluminum.

Trains are heavy and often operate at faster speeds than buses, that's why they can get away with relatively few axles.

https://sfbayca.com/2016/05/05/new-muni-trains-on-schedule-f...