If the choice is between safety and aesthetics, or safety and convenience, there really isn't any choice at all. That's how we view the news that the FasTracks commuter trains at Denver Union Station will arrive and depart at ground level rather than be funneled through an underground facility behind the historic railway station, as everyone had long assumed.

Still, the last-minute change in plans is stunning - and baffling, too, for reasons we'll explain.

For years, a below-grade commuter station was part of the Union Station redevelopment concept. The city wanted an underground platform and the Regional Transportation District agreed to the goal, too.

The two finalists in last year's competition to redevelop the site both offered plans with a below-grade station for commuter rail. In fact, the winning plan was somewhat controversial only because it didn't bury the light rail station, too, locating it instead at street level farther into the Platte Valley.

In other words, the goal of underground commuter rail was not exactly a state secret. And yet RTD apparently never realized that the federal agency needing to sign off on the facility's safety - the Federal Railroad Administration - had absolutely no intention of doing so.

What gives? Have these two agencies been communicating with semaphore flags? By smoke signals? Are their offices still not equipped with phones, faxes and e-mail?

More seriously, had federal officials been dropping hints of disapproval that RTD never picked up - or, more disturbingly, chose to ignore? Or had the feds simply neglected to clearly convey their dismay with the plan?

Whatever the explanation, it doesn't speak well about the Union Station planning process.

It wasn't until last month, incredibly, that the railroad agency's administrator warned RTD in no uncertain terms that it had better drop plans to bury commuter rail or expect an order "prohibiting trains from using the station."

The tone of the letter from Joseph Boardman to Cal Marsella, RTD's general manager, is astonishing. It suggests federal officials were actually shocked that RTD would seriously intend to build a "stub-end station" - one in which the tracks dead end - that would require approaching trains to slow down while proceeding downhill. "To the best of our knowledge," Boardman wrote, "no one has ever suggested converting a through-station like this in the United States to a stub-end station located at the bottom of a steep downgrade. . ."

Why not? Because of the "the realization by railroad engineers that a train or an individual rail car could cause significant damage to people and property if either overran the end of the track, due to mechanical or human failure." Indeed, Boardman noted, "there have been many instances where passenger trains have overrun through-stations by hundreds of feet. And, "while instances of running into structures at the end of stub tracks are far fewer . . . the consequences are far greater . . ."

Indeed, Boardman added, in prose that seemed to reflect growing exasperation, "we have seen references that indicate the [United Kingdom] experiences approximately 20 collisions a year with end-of-track devices at stub-end stations on predominantly level track."

Although we've supported the underground concept, we've got to admit that Boardman makes a strong case. For that matter, it's refreshing to see a federal official write a letter whose meaning is so unmistakable and whose commitment to protecting the public is so apparent.

The only question is, why on earth wasn't this letter delivered months or even years ago?