SAN CARLOS — Caltrain passed a key milestone Thursday in its $1.5 billion project to electrify its railway, unveiling the final environmental impact report on an ambitious undertaking that will allow the agency to boost ridership, reduce air pollution and prepare for the arrival of high-speed rail.

The agency debuted the report in a presentation to its board of directors, which is expected to approve the document next month. At that point the agency can hire a team to design and build the new system.

The plan calls for erecting poles and wires above 51 miles of track from San Francisco to San Jose, installing power supplies along the rail corridor and purchasing a new fleet of trains. Caltrain anticipates construction beginning in 2016 and finishing by 2021.

The agency received nearly 230 comments on the report, which is designed to ease potential impacts of the project on the environment and surrounding community.

Concerns included how the new system will look, effects on local vehicle traffic, ﻿easements and property seizures to accommodate the poles and power stations and the number of trees that will be trimmed or felled to make room for the poles.

Caltrain’s growing number of bicyclists are clamoring for more room to store their bikes. Those decisions won’t be made until the agency purchases its new train cars, but Caltrain board member and San Mateo County Supervisor Adrienne Tissier said she’s sympathetic.

“I do think we need to come up with greater capacity for bikes,” Tissier said after Thursday’s meeting, while striking “the right balance with the riders.”

The Peninsula Corridor Electrification Project will enable Caltrain to boost its ridership from roughly 60,000 today to more than 110,000 by 2040, according to the agency’s projections. The increase will come in part from the new power source. Electric trains accelerate and decelerate faster than diesel trains, allowing for more trains per hour.

But the agency may need to take extra steps to accommodate riders, such as increasing the length of trains, said Adina Levin, executive director of the nonprofit Friends of Caltrain.

“We are concerned that the ridership forecast is unrealistically low,” said Levin, noting Caltrain’s ridership has more than doubled since 2003. “More planning will be needed to keep up with ridership growth.”

Caltrain spokeswoman Jayme Ackemann said the agency is already beginning to study those issues.

Two key advantages of the new system relate to noise and air pollution, Caltrains says. The new trains will be quieter and cleaner than current models.

The agency also predicts the switch from diesel to electric power will remove 176,000 metric tons of carbon dioxide from the atmosphere.

Ackemann said Caltrain is committed to pursuing mitigation measures outlined in the environmental report. But the agency also says that, if the environmental report is challenged in court, it has the authority under federal rules to claim an exemption from the California Environmental Quality Act. Ackemann said this is merely an option.

“Caltrain has no option at this point but to move this project forward as expeditiously as possible,” she said. “Right now many of our trains are running at 120 percent capacity or more.”

Electrifying the Caltrain corridor will allow the agency eventually to share the system with the California High-Speed Rail Authority, which has committed to spending $705 million on Caltrain’s electrification as well as new signaling and train control systems.

Contact Aaron Kinney at 650-348-4357. Follow him at Twitter.com/kinneytimes.