There are three stops on the Tram:
- Lower A gates (1 - 26)
- Center of Terminal A
- Upper A gates (55 - 78)
Trams hit the center station about every 2-3 minutes. Anything between 27 - 54, you are probably better off walking. There are plenty of moving sidewalks, and you could, if you walk quickly, make it end to end in under 15 minutes.
Information on the DTW McNamara Terminal -
http://www.metroairport.com/services/mcnamara.asp