SAN JOSE — The BART to San Jose extension has entered an essential phase, with testing of trains on the 10-mile link from Fremont to the Berryessa area set to begin in a few months.

The Valley Transportation Authority, which is overseeing construction in Santa Clara County, has electrified the third rail that will power future BART trains.

Unlike VTA’s light-rail trains, which are powered by overhead lines, the BART system consists of two high-voltage substations, six traction-power substations, and contact rails. The contact rails that run alongside the train tracks, known as the third rail, will be powered by 1,000 volts of electricity.

High-voltage paddles on the underside of the BART trains come in contact with the third rail, feeding the trains the power necessary to run along the track.

But with the electrification comes a stern warning from John Engstrom, VTA’s BART project manager.

“You can’t see the current, you can’t smell the current, but if you touch the current, it will be too late,” he said. “Now that we are in the system-testing phase, the site becomes extremely dangerous.”

Just like the system to the north in the East Bay, the Berryessa extension will be fully access-controlled. It has 8-foot-high fencing with barbed wire and sound walls along the corridor. All gates are locked, and there are “Do Not Enter” and other various safety and warning signs placed throughout the alignment.

“Every person in the public needs to understand that the BART right of way is not a shortcut,” Engstrom said. “It is not accessible for a reason — for people’s protection.”

The $2.3 billion extension broke ground in April 2012. The VTA hopes to begin carrying riders in late 2017.

Next month, Milpitas Boulevard between the Montague Expressway and Garden Street will be closed for five weeks to raise the intersection and match the grade of the new bridge being installed at Montague. Underground utility work will also be completed during this closure.