The MTA is doubling up.

New York City Transit will be testing its first double-decker commuter bus in decades, agency officials announced Monday.

The 12-foot, 9-inch blue bus will begin running from Staten Island to Manhattan sometime in the next few weeks — but Transit head Andy Byford plans to add more across the system if this one is a hit.

The city ran double-decker commuter buses from the ’30s to the ’70s and has toyed with bringing them back since but they’ve never caught on for a variety of reasons. There are, however, fleets of double-decker tour buses throughout Manhattan.

The inside of the bus is tight, with one staircase to the upstairs. It’s meant for longer routes with shorter stops and wouldn’t work well for local bus trips, said Darryl Irick, head of the MTA’s bus division.

The bus will run on the 17J express route, which takes riders from Staten Island into Manhattan via New Jersey and the Lincoln Tunnel.

Though Byford cut his teeth in London’s transit system, it was not his idea to add the double-deckers here. He took over NYC Transit in January and the MTA ordered the bus about eight months ago, said agency officials.

The double-decker is part of Byford’s plan to completely overhaul the system, however.

The plan will also include reconsideration of every bus route in the system, as well as a path toward all-door boarding once the MTA’s new payment system is in place, officials said.

Byford’s bus plan is modeled after MTA Chairman Joe Lhota’s Subway Action Plan, which he released last summer to try to stem massive delays in the system.

The bus plan’s main goals are to make buses faster and more efficient, as well as bring riders back into the system, officials said.

“This is really is about tackling long-standing issues,” Byford said. “We want to make this a reality and get people back on the buses.”