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.