BOSTON (AP) _ Workers returned to their jobs Monday at one strikebound railroad and job openings were posted at two others after President Reagan's intervention in the 2 1/2 -month-old walkout.

Guilford Transportation Industries, which owns all three lines, said it would lay off some of the workers returning to its Maine Central Railroad and would not rehire all strikers on its two other lines because business was down 30 percent due to the strike.

The company's Boston & Maine Railroad was inspecting tracks and signals Monday in preparation for resuming full commuter rail service, possibly as soon as Wednesday, said Guilford Vice President Colin Pease.

Commuter rail service the B&M operates under contract for the Massachusetts Bay Transportation Authority has been curtailed by the strike on four lines from Boston to its northern and northwestern suburbs.

The strike began March 3 by 123 track workers on the Maine Central. It soon spread to the B&M and Guilford's third railroad, the Delaware & Hudson, and involved up to 3,500 workers, according to the Billerica-based parent company.

Reagan intervened in the strike Friday when it spread to the sprawling Conrail system and threatened to hamper rail transit in the Northeast. He ordered formation of an emergency panel to settle the dispute and called for Maine Central workers to return to work for a 60-day cooling off period.

The company had abolished the job positions held by the strikers on the B&M and Delaware & Hudson lines, said Guilford spokesman Dennis Coffey. The company would post many of the jobs as new openings but did not plan to rehire all striking workers because of a downturn in business, he said.

The returning workers and the 700 temporary workers hired during the walkout could apply for the openings, he said, but since the jobs would be filled according to seniority, few temporary workers would be retained.

The rehiring plan was challenged Monday by Robert Naumes, a lawyer for the Brotherhood of Railway Maintenance Employees, the union that originally struck the Maine Central. He said he interpreted Reagan's order to cover all three Guilford lines.

''It's our position that all of the unionized work force on all three railroads should be put back to the position they were in prior to the Maine Central strike,'' he said.

Naumes said he would ask a federal court in Portland, Maine, to order Guilford to rehire all returning workers on the three lines. He said most legal filings in the strike have taken place in Maine.

Unionized workers returned Monday at key rail yards in Maine, said William LaRue, international vice president of the union.

Striking workers trying to return to their jobs at the B&M, Maine Central and the D&H were turned away Saturday, Naumes said.

The initial strike by 123 workers against the Maine Central centered on job security, wages and severance benefits.