Before every major congressional recess, Capitol Hill doubles as a rail yard.

The parliamentary conductors always run the usual trains up and down the congressional tracks. Pieces of legislation that will likely pass but might not reach their final destination. For instance, the House of Representatives is set to run out a bill Thursday to penalize cities that don't adhere to federal immigration policy.

Republicans might like it. Democrats don't. And President Obama would never sign it.

But there's always at least one, major express train, sitting in the roundhouse. This is the single train designed by the congressional yardmasters to run its route. The final train out of the session. The month-long, traditional August recess starts soon. And if you want to get your legislative item loaded as cargo, lawmakers know they need to add that freight to the manifest.

If Christmas has the Polar Express, then summertime on Capitol Hill features the August Recess Express.

Ironically enough, the train leaving the station before this August vacation involves another mode of transportation: the nation's highways.

The Highway Trust Fund is expected to run dry at the end of the month. The fund pours tens of billions of dollars into major transportation projects around the country, paid for by an 18.4 cent levy on each gallon of gas. But the Trust Fund is now in the red because gas tax revenues fall short. Congress routinely approves a patch to make up the difference. But a failure by Congress to re-up the fund could halt thousands of construction projects in the middle of the summer and throw tens of thousands of workers off the job.

The House of Representatives approved a stopgap, five-month extension of the Trust Fund last week. But Senate Majority Leader Mitch McConnell, R-Ky. -- alongside Senate Environment and Public Works Committee Chairman Jim Inhofe, R-Okla., and Sen. Barbara Boxer, D-Calif. -- crafted a six-year, partially paid-for highway bill.

The Senate initially stumbled Tuesday to clear a procedural hurdle just to launch debate on the bipartisan plan. Many senators from both parties balked at starting debate on a 1,030-page bill they hadn't yet studied. But the Senate managed to narrowly round up the necessary votes to advance to the plan Wednesday night.

So the train is on the tracks. It's just a question now as to whether that locomotive manages to leave the station and what constitutes its cargo.

"I still stand behind what we passed," said House Majority Leader Kevin McCarthy, R-Calif. "I think the Senate should take up the short-term bill."

The problem is that House Republicans balk at the Senate bill. They argue they had no say in the Senate legislation. House Democrats object to the revenue streams to offset the cost of the bill.

So why is McConnell insisting on a longer-form plan instead of the House's stopgap? It's likely lawmakers will have to approve an interim spending bill to avoid a government shutdown in October. Such a plan may only run until mid-December before congressional appropriators must craft a so-called "omnibus" plan to run for the remainder of the fiscal year -- Sept. 30, 2016. Then Congress must wrestle with raising the debt limit around the holidays. Having the Highway Trust Fund ripen simultaneously is a toxic trifecta which McConnell and others don't want to endure at Christmas.

Thus, McConnell is working to advance a longer bill in the Senate over the next few days. But then there's the issue of what payload lawmakers stash on the August Recess Express.

The worst-kept secret in Washington is that backers of the now-expired Export-Import Bank (a government institution which grants loans to U.S. corporations to conduct business abroad) will attempt to attach an amendment to the highway bill. The amendment would re-establish the Export-Import Bank's charter. A coalition of some Republicans and many Democrats would likely pass a renewal of the Export-Import Bank in the House and Senate.

Then there are other parcels, resting on the congressional platform.

Sen. Rand Paul, R-Ky., is promising an amendment to the bill to slash federal funding of Planned Parenthood. The organization is under fire after undercover video showed its employees discussing the business of harvesting fetal tissue and organs.

But no one is clear whether the Senate can even approve the longer-term highway package. What the Senate can pass depends on the final waybill -- indicating whether Planned Parenthood or immigration or a host of other issues made it onto the train. The prospects of adopting such legislation in the House are sketchy at best.

Plus, there is always the House's five-month highway bill. That train is chugging down the tracks. If the Senate's measure derails, it's likely senators will drop back to the House plan.

That wouldn't be a very long freight train. But the House's interim highway measure would be the only train of consequence leaving the station.

And it would bear the moniker the August Recess Express.

Capitol Attitude is a weekly column written by members of the Fox News Capitol Hill team. Their articles take you inside the halls of Congress, and cover the spectrum of policy issues being introduced, debated and voted on there.