While Congress is often stalled or bitterly divided in dealing with some of the biggest issues facing the nation, its members are often remarkably successful in promoting industries or more parochial concerns. Mr. Mica’s championing of the SunRail project has won praise among many officials and business leaders here, who say it is vital to the future of the traffic-clogged region.

But skeptics question whether Mr. Mica’s real goal is to give a taxpayer-financed gift to CSX, the freight rail giant and a generous Mica campaign donor, which would get $432 million for its tracks and for upgrades to tracks it owns elsewhere in the state. Other Florida businesses close to Mr. Mica also stand to benefit if the project is built.

“His dedication to SunRail is not for mass transit — it is for helping CSX to get government funds for its private freight lines,” said State Senator Paula Dockery, a Republican and a chief critic of the project.

The congressman is unapologetic about SunRail. “Everybody has different vested interests,” Mr. Mica said in an interview. “But you look at what is being proposed on paper, and it just make sense.”

Mr. Mica first proposed the commuter system in 1992 after he won his first race for the House after a career that included stints as a real estate developer, a lobbyist and chief of staff to Senator Paula Hawkins, Republican of Florida. The CSX track, which for decades has also been used by Amtrak, already connects a string of communities north and south of Orlando. Commuter rail, Mr. Mica argued, could carry many more passengers and be far less costly than adding a single lane to an Interstate highway.