Pittsburgh, USA

The Free Fare Zone was established to promote the use of transit within Downtown Pittsburgh, encourage intramodal transfer and reduce boarding delays.

T Free Fare Zone

Rides within the T’s Free Fare Zone are free at all times. The zone includes the following stations:

Downtown Pittsburgh: First Avenue Station, Steel Plaza, Wood Street Station and Gateway
North Shore: North Side Station and Allegheny Station
Free rides to North Shore stations are provided thanks to sponsorship agreements with the Pittsburgh Steelers, Rivers Casino, ALCO Parking and the Stadium Authority of the City of Pittsburgh.

Bus Free Fare Zone

For the bus, the Free Fare Zone is defined by the following boundaries:

The Allegheny River
The Monongahela River
Ross Street, Penn Station and Liberty Avenue at the railroad overpass and 11th Street
For bus riders, fares are free within this zone from start of service until 7:00 p.m., seven days a week.

More info here.