Enterprise at SFO
 

I notice there is a offsite enterprise rent a car location pretty much across the freeway from the SFO car rental agencies.

Renting from that offsite location is Significantly cheaper due to the airport fees.
Its location is At 734 San Bruno Ave E, San Bruno, CA 94066

has anyone rented from there?

Can you take the AirTrain and get off at the normal SFO rentals and then walk to the off site location?
and assuming return can be done the same walk back to the AirTrain after dropping off the car? There’s no restrictions on getting on and off the AirTrain if I’m not renting from the on site rental agencies?

IIRC the AirTrain is just an open-access people mover, so you shouldn't have a problem taking it and then walking. It's also not far at all for an Uber.

I've rented there when my car has been in the body shop in the area when paid by insurance. Decent service but limited inventory. It's a good hike if you walk from the onsite rental buidling.