I'm unsure about the Detriot specific case (it might have an unusual number of road crossings on very high traffic streets) but I'd suspect it would reduce congestion by taking cars off the road. Most commuter trains have this effect.
There's also Braess' Paradox to be aware of, which means that reducing congestion can actually INCREASE congestion. https://brilliant.org/wiki/braess-paradox/
My comment left the possibility of Brasess' Paradox open in the specific Detroit case, but practically speaking commuter trains are used by most major cities worldwide as part of a comprehensive transportation system. For many cities rail is among the most efficient subsystems of their transportation system.