Mayor Richard J. Daley engineered Chicago's most prolific era of massive construction projects. Generous influxes of federal money helped build this city's runways and expressways. Subsequent mayors have yearned to provide the same sense of ever-expanding infrastructure — and to prompt the hiring of Chicago construction workers by the thousands. Last week, speaking to an audience at a West Side union hall, Mayor Rahm Emanuel revived talk of one such potential project: high-speed rail service from the city's downtown to O'Hare International Airport. "To connect people to O'Hare even faster, we're going to embark on a project that has been imagined and discussed for decades, but is essential for our city's future," Emanuel said.

We approach Emanuel's exhortation with an open mind but also with fundamental questions, starting with: Why, exactly, is this service to and from O'Hare essential to Chicago's future? Would customer demand generate sufficient patronage to justify the comparatively high fare? Judged against existing (and potentially improved) Blue Line service, by how many minutes could rail service actually shorten a traveler's trip door to door? As you mull that, bear in mind that riders who now can board those CTA trains at multiple stations across central Chicago presumably would lose time traveling — via CTA or auto or on foot — between their Loop locales and a single downtown train station.

Maybe the Emanuel administration will have compelling answers to these and other, subsequent questions about costs and timing. Chicago has gotten behind other projects — think of the ongoing modernization of O'Hare — but only after City Hall devoted considerable attention not just to the "what" but, first, to the "why." Talk of big construction projects can dazzle citizens, but only for a while. It took rivers of data on passenger and flight projections, on comparable projects at other airports, on the very future of global aviation itself, to convince Chicagoans that they should bear the burden of retrofitting O'Hare for the coming decades.

We're proud to have been among the voices that, once persuaded, strongly supported that project.

We're also enormously proud of the global-city status that Emanuel often invokes when he speaks of Chicago. And we're patient; City Hall doesn't have to have answers to everything before a mayor floats a proposal.

All of that said, appeals to civic pride or bragging rights don't, by themselves, justify what surely would be a disruptive and costly project. Other proposals have foundered — remember the transportation superhub that would undergird the Loop's Block 37? — because civic officials across this metropolis didn't have good answers to the kinds of basic questions we're raising. An edifice complex can't justify mega-projects, no matter their initial appeal.

What's the genesis of this proposal?

Emanuel was correct when he said that talk of a better-than-CTA rail link to O'Hare has been "discussed for decades." Two possible routes to and from O'Hare that were envisioned: the existing Blue Line right of way — perhaps with a towering double-deck structure — or the Metra North Central Service corridor, which runs between Union Station in the West Loop and Antioch, with a stop at a transfer station at O'Hare.

In 2008, amid more than $100 million in cost overruns, the CTA abandoned plans to run express trains from the planned superstation below Block 37 to O'Hare and Midway airports. By that time, the project already had cost more than $200 million, most of it public money. But Mayor Richard M. Daley never really let go of the idea. In 2010, he rode a Shanghai bullet train that rocketed at speeds up to 267 mph, an experience that led him to explore having private investors finish the project and run the system. And then he retired.

We'll stipulate that some travelers would pay a high fare for more comfortable, more spacious and more speedy service — assuming this project actually can deliver those perks — to and from O'Hare. Just as we'll stipulate that making the journey today on the Kennedy Expressway often isn't an express experience.

Which isn't to say, though, that major updating or reimagination of Blue Line service can't deliver a much improved experience between the airport and downtown at a fraction of the expense and inconvenience to the very travelers Chicago wants to serve.

Does high-speed rail service to O'Hare make sense? We appreciate your call to action, Mayor Emanuel, but we also await your hard evidence.

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