[inferiorhuman]

  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.