For years, Caltrain officials have been advocating a switch from diesel trains to electrified ones as the the best way to both help the environment and keep the popular but cash-strapped commuter service financially viable.

But a new report analyzing the environmental impacts of electrification indicates that these benefits will come at a cost beyond the project's $1.5 billion price tag. Specifically, it could result in removal of more than 2,000 trees and the addition of poles up to 50 feet high, safety walls built on existing bridges that cross the train corridor, and substations -- including one in Palo Alto -- to support the electrification.

The draft Environmental Impact Report, which the Peninsula Corridor Joint Powers Board released Friday morning, argues that Caltrain's long-planned electrification is a critical project for increasing ridership and for giving the Peninsula an "environmentally friendly and reliable service." More than a decade in the works, the previously stalled project sparked back to life in 2012, when the California High-Speed Rail Authority agreed to adopt a "blended" two-track system along the Peninsula in which the new high-speed trains would share electrified tracks with Caltrain. As part of a 2013 agreement between the agencies, the rail authority would pay for about half of the project's $1.5 billion costs, with the balance coming from Caltrain and other Bay Area transportation agencies.

According to the new report, Caltrain plans to have its new electrified system in place by 2019, at which time about 75 percent of its train fleet would be electric and 25 percent would be diesel. Once the remaining diesel trains reach the end of their service life, they would be replaced.

Caltrain carried about 47,000 riders on a typical weekday in 2013, according to the report, a number that is projected to go up to 57,000 in 2020 and to 84,000 even if electrification doesn't happen. With the project, the estimated ridership would be 69,000 in 2020 and 111,000 in 2040. The overall number of daily weekday trains would jump from the present level of 92 to 114.

The environmental review notes that the project would significantly reduce traffic on regional roads by 235,000 "vehicle miles traveled" in 2020 and by 619,000 in 2040.

Yet the benefits will come with costs. The overhead power lines would be supported by poles with heights ranging from 30 to 50 feet, according to the report. The poles would stand on either side of the tracks, about 10 to 12 feet from the centerline, and would be spaced about 200 feet from each other (with shorter spans between poles on curved track sections). Wires would stretch across the tracks in a cantilever configuration.

The electric infrastructure would also require installation of one switching station, which controls how power is fed within the system; 10 traction power substations, which convert electricity to the voltage trains use; and six paralleling stations, which boost power along the system.

One paralleling station would be in Palo Alto, either near Greenmeadow Way and just south of Page Mill Road, according to the report. But, the report notes, such a station would have some visual impact. Located in a compound that has typical dimensions of 40 feet wide and 80 feet long, the station could be partially screened by trees. If located by Greenmeadow, "roadway users and residents may have limited views" of the site when there are gaps in vegetation.

The Page Mill option would also benefit from screening provided by trees on the Alma Street side and from the new four-story Park Plaza building on the other side, according to the report.

The environmental analysis noted that the Greenmeadow Way option would require trees to be removed, causing "significant" aesthetic impact. Caltrain is proposing to compensate by installing new "screening vegetation" along Alma between the roadway and the new station.

In addition to the electric infrastructure, Caltrain plans to build safety barriers on dozens of existing bridges to prohibit access to the Caltrain corridor and to prevent objects from being thrown off the bridges, according to the document. These barriers would typically be about 6.5 feet above the pavement level and would generally be about 40 feet long. Each barrier would feature a black, red and white signage that reads: "Danger. Live Wire."

The 47 bridges identified in the report include one bridge in Palo Alto (two new walls would be built on the San Antonio Road overpass) and six in Mountain View (Shoreline Boulevard overpass; Stevens Creek pedestrian crossing; Whisman Road; Route 85; and Route 237, both eastbound and westbound).

While the new infrastructure will be going up, hundreds and possibly thousands of trees would be going down. The report estimates that about 2,220 trees would be removed for the project and another 3,616 pruned. This includes 188 trees in Menlo Park, 177 trees in Palo Alto and 284 in Mountain View, which is second only to Sunnyvale's 497.

The report notes that Caltrain is exempt from local regulations guarding tree removal because it is a federally regulated rail carrier and thus benefits from an exemption in the Public Utilities Code. Still, it lays out a strategy to mitigate the loss of trees, including locating poles and alignments to "minimize tree removal and pruning" and removing trees "only as necessary to provide safety clearance." The project would include a creation of a "Tree Avoidance, Minimization and Replacement Plan," which would be developed in consultation with cities and an certified arborist and which would consider best practices for replacing and protecting trees.

The report is subject to modification based on comments from stakeholder communities along the corridor. But Caltrain officials stressed the importance of releasing the document, which Caltrain Executive Director Michael Scanlon called "the next step in a critical partnership between Caltrain and the communities we serve."

"We must work together to ensure the successful delivery of the Caltrain Modernization Program," Scanlon said in a statement. "We are committed to seeking public comment and to make sure the concerns of our communities are addressed directly, collaboratively and transparently."

Caltrain will be accepting comments on the draft EIR until April 29. The document can be found here.