The House voted yesterday to reduce funding for Amtrak next year by about 11.5 percent, to $603.5 million, and to create a commission to seek ways to increase the passenger rail system's income.

The legislation also requires Amtrak to continue operating all its current routes, even if it must use capital funds that otherwise would be used for new equipment or property improvement.

The Amtrak cut was approved by voice vote. The overall bill, which reauthorizes Amtrak operations next year, was approved, 290 to 128, and sent to the Senate.

President Reagan, in his fiscal 1986 budget plan, proposed the elimination of all funding for the politically popular Amtrak system. But Congress balked and, in the budget resolution approved in August, proposed a 15 percent decrease as part of a $55.5 billion deficit reduction package for next year.

Yesterday, Rep. Robert S. Walker (R-Pa.) said the Democratic-controlled House's failure to meet this goal was one more sign that the House was determined to find "all kinds of mechanisms to get around that budget" resolution.

Last week, during consideration of the Department of Transportation spending bill for next year, the House defeated an amendment to bring Amtrak funding in line with the budget resolution, agreeing instead to an 11.5 percent cut.

The rail system, which received $684 million this fiscal year, would be reduced to $603.5 million in fiscal 1986, which begins Oct. 1, if yesterday's House vote survives.

The 15-member commission established by the bill would seek to determine whether Amtrak can improve its financial position. Amtrak, which serves about 20 million passengers a year, has failed to become the self-supporting, for-profit corporation that Congress envisioned when it created Amtrak in 1971.