Vice President Biden, Transportation Secretary Ray LaHood, and Sen. Ben Cardin (D-MD) will be appearing at a suburban Maryland train station tomorrow morning to tout the congressional stimulus bill -- or in the White House press office's words, "the need to invest in transportation infrastructure in order to build a 21st century economy." And few thinking Americans would challenge them on that point.

But as lawmakers and the mainstream press are coming to realize, and as we noted weeks ago, the stimulus plan dedicates stunningly few resources to creating the type of transportation infrastructure that can alleviate over-taxed public transit systems while weaning the nation from its obsession with environmentally unsustainable car travel.

What's the trouble? Why aren't we seeing liberal Democrats, at the very least, push for the kind of groundbreaking transit projects that not only create jobs, but fulfill the president's promise for a massive investment in public works?

Part of the answer lies in two parallel transportation policy story-lines that are playing out on the Hill this week: one dealing with Senate environment commitee chairman Barbara Boxer (D-CA), the other with what advocates call the "fix-it-first" requirement.

Let's take Boxer first. Green transit advocates are perplexed by her decision to sign on to a amendment sponsored by mass transit critic Sen. Kit Bond (R-MO) that would strike a $5.5 billion surface-transportation grant program and instead use the money for general road-building.

What's even more distressing is the possibility that Boxer could sign on to a proposal from her GOP counterpart on the environment panel, the notorious climate-change denier Sen. Jim Inhofe (OK), to divert up to $50 billion in unspent stimulus money to road-building.

The Inhofe plan hasn't materialized yet, and my inquiries to Boxer's staff on the question haven't yet been answered. But as Environmental Defense Fund transportation director Michael Reflogle told me, "this massive shift to the highways account would do nothing to enhance the system of public transportation and come with no assurances" that the money would be used first on repairing existing roads before building highways that may not be needed.

This is what's known as a "fix-it-first" requirement -- the second of my two story-lines -- and it's something that Congress has proved sadly unwilling to include in the stimulus bill. "Intense pressure has been put on senators" to pass the stimulus quickly, Reflogle explained. "We're certainly hoping that what ultimately gets considered might look different than what was discussed initially, might reflect some of the concerns we raised."

But why would the liberal Boxer would reach out to the conservative Inhofe on building new highways on the same day that he publicly trashed her principles for climate change legislation? Meanwhile, Inhofe was telling Fox News the following about voting for the overall stimulus bill:

Well, let me, first of all, just say I'm so proud of the Republicans in the House of Representatives. It's the first time in my memory that all -- all of them got together and did the right thing. I would like to see that happen in the Senate.

Californians have long been speculating about a Senate matchup in 2010 between Boxer and Gov. Arnold Schwarzenegger (R) -- meaning that the sitting senator's ability to bring home the bacon for her state may ultimately matter more than whether the bacon goes toward roads or trains. But if a Democrat with Boxer's cred isn't pushing for a "fix-it-first" requirement in the stimulus bill, who will?

Kate McMahon, a transportation policy campaigner for Friends of the Earth, lamented the lack of a constituency for fix-it-first as "atrocious."

"We have 20, 30 percent of our highway infrastructure that's structurally deficient, and we're not going to direct the money toward that, or at least have some sort of line in there that says we 'should give preference' [to repairs]?" she asked.

When I wondered aloud about the political climate that would make such a seemingly practical provision out of reach, she observed: "The road-building lobby is pretty powerful. People just don't want ... to deal with it. At this point, [fix-it-first] would have to be something that gets voted on, and no one wants to offer an amendment that won't pass. Right now they don't even know if they can get the package passed entirely."