Beyond the danger of snarling downtown traffic, the surface line would duplicate service provided by buses and the Market-Frankford subway.

Another big-picture concern is that any plan to run trolleys up and over I-95 would saddle the waterfront for decades with the ugly, existing scissor ramps leading down to Columbus Boulevard.

Straddling I-95 with a new rail bridge might also deter city officials from pursuing a better solution for the highway: to bury or cover it. As long as I-95 stands as a barrier to Center City, it will complicate and possibly stymie efforts to create the thriving waterfront envisioned by Mayor Nutter and city planners.

There are other problems as well: The projected ridership is low. As such, the project fails to qualify for federal funding. At an estimated cost of $500 million, a big question is where will the money come from?

Delaware River Port Authority officials are banking on the city's congressional delegation to change the funding formula in Washington in order to qualify for federal funds. That's not a slam dunk.

For now, DRPA is moving forward with its plan, announced last week, to use Market Street as the Center City link to the waterfront. The next stage is a two-year process during which DRPA staff plan to do preliminary engineering and environmental studies.

In the first stage, trolleys would run along tracks in the middle of Columbus Boulevard from Pier 70 north to Girard Avenue, linking two casinos planned for the waterfront. It is hoped that the project will be more than a casino run.

Any trolley service should be part of the master plan for the waterfront that's being developed by the Delaware River Waterfront Corporation. The design of the route into Center City should be coordinated with that planning effort. If that means delaying the rail line, then bus service could be used in the interim.

It would make more sense for riders to reach Second Street on the El, then transfer to the waterfront line - preferably underground. That would meet the goal set by Nutter's transportation deputy, Rina Cutler, to tie in the new line with the main transit system.

To his credit, DRPA chief John Matheussen says his agency will look at using the subway line along Market Street. Since there's no federal money yet for this project, there's ample time to get the design right.