Service dogs on trains and transit
Air travel is not the only mode covered. U.S. Department of Transportation rules protect your service dog on ground transportation too.

Photo: Mike Kilcoyne / Unsplash
The rule
Under 49 C.F.R. section 37.167(d), transportation providers must permit service animals to accompany individuals with disabilities in their vehicles and facilities. That covers rail, bus, light rail, and public transit.
What it means
You board with your service dog in the passenger space. Providers may not require special documentation for transit in the way an airline may for a flight, and they cannot charge you extra for the dog.
One standard everywhere
A dog individually trained to tasks and steady in public is positioned correctly across air travel, ground transit, and public access. Train once, travel everywhere.