By John Crawley

WASHINGTON, Aug 6 (Reuters) - Congress could soon nudge the door open to private investment in U.S. passenger rail, once abandoned as a money loser but now drawing attention with ridership increasing and federal subsidies expanding.

A provision in a bill to continue subsidies for national provider Amtrak would direct regulators to seek proposals for designing and operating high-speed rail between Washington and New York.

Proponents believe halving the nearly four-hour trip would more closely rival trips by air and make downtown-to-downtown trains more attractive to business travel -- and outside investment.

If the response is positive, the government would consider a pilot program in the Northeast and weigh options for investment in other rail projects.

Rep. John Mica, a Florida Republican and architect of the plan, said in an interview that he has met with U.S. and international finance interests and transportation companies to gauge their views.

"They (investors) are watching to see what Congress will do. If we open the door and don't put too many impediments to competition, you could attract significant investment," Mica said.

Companies that participated in a roundtable on high-speed rail hosted by Mica, or gave him promotional materials, include Virgin Rail, a unit of Virgin Group [VA.UL], and Germany's Siemens (SIEGn.DE: Quote, Profile, Research). Virgin Rail is in a partnership to run service in Britain while Siemens is a global manufacturer of high speed trains.

There is some resistance in the Senate to Mica's plan, which is a cornerstone of rail legislation approved by the House of Representatives, but not included in a similar Senate bill. Negotiators from both houses will address the matter in September but it is unclear if a compromise will emerge.