From the Metrotimes:

But a distinction in the authority’s bylaws was raised by proponents of rail transit before the state legislature ever gave the bill final approval.

The RTA’s bylaws show a clear distinction, and, some say, preference to bus options over rail. The authority’s board needs a supermajority to ask voters to approve a tax that would fund future operations and capital costs for the BRT system.

But to construct or operate a rail line, such as the proposed M-1 streetcar line in downtown Detroit, which officials hope to eventually turn over to the RTA, the board requires a unanimous vote.