The downtown Seattle transit tunnel is an unusual beast; it was built from the start as a hybrid bus and rail system, though the city did not yet have any plan to build a train network. The builders designed the tunnel and its stations for train service and laid tracks anyway, expecting they would eventually be needed.

So... where BRT is a proposal to save money on transit by building cheaper infrastructure - a promise it can always keep, since bus service can be degraded as far as necessary to fit a budget - the DSTT project instead used the promise of future rail service as a motivation to spend more money on transit, building better infrastructure than they actually needed at the time.