Check out OpenSignal app, they have the most detailed coverage map.
I have the same problem with T mobile. No reception while sitting in Caltrain around the 4th and King station. Frustrating if I arrive early, sit in my train waiting for it to leave and round that corner.
I never had any problems around that area and my carrier is at&t. I perform a test on Ookla app and it was 110mps.
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