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Most passenger US rail systems have APIs with realtime positions but I don't know of a single map that includes all of them in the whole country.

Here's a live one for Boston: https://tmap.live/

Here's a live one for NYC: https://aptransit.co/

For your son though just get the "Transit" app by CityMapper

Amtrak has a 'track your train' portal that shows you where all the trains are

The location updates, esp. in open areas, is a bit delayed probably due to poor cell tower access :-)

I recently tried to find something like this for the US but it appears security concerns limit public access to a realtime map like this.

They should classify it as personal information, and you could share, sell, and publish it as much as you want.