Transit vehicles purchased with federal funds are required to meet a minimum service life (see below). For the Muni Metro you're looking at a minimum expected life of 31 years.
That's an average service life from what I can tell, not a minimum (a minimum average?). But yes, it does trend a tad higher than what my cursory search indicated (25-30, feds want 31)
Looks like this has been the subject of some industry wandering in the last generation:
Some were planned for 30-35 years, some for 25 to 30, some models have aged so gracefully they may be refurbished, yet others may have been engineered with roughly the correct obsolescence window in mind as engineering has advanced in the last generation, even in the dimensions of making some things simpler and more reliable (one of the billed advantages of some of the Siemens train mechanisms).
FTA has set a default ULB as the expected service years for each vehicle
class in the table below. … In cases where the submitted ULB differs
significantly from the default value, agencies may be prompted to submit
justification.
The first generation of modern streetcars (the Boeing-Vertol) barely made it 24 years and that was scandalous. The next generation (Breda) were so bad that their manufacturer was banned from city contracts. Even so the Boeing cars were retired post haste as Muni took deliveries of the Breda cars. The last of the Boeings were decommissioned in 2001, and the Bredas look like they'll be around the full 31 years at least.
Basically if you're buying things that don't last 30 years, the feds are going to be very reluctant to give you money.
Looks like this has been the subject of some industry wandering in the last generation:
https://www.urban-transport-magazine.com/en/new-boom-in-the-...
Some were planned for 30-35 years, some for 25 to 30, some models have aged so gracefully they may be refurbished, yet others may have been engineered with roughly the correct obsolescence window in mind as engineering has advanced in the last generation, even in the dimensions of making some things simpler and more reliable (one of the billed advantages of some of the Siemens train mechanisms).