Investor interest in infrastructure stocks zoomed after President Donald Trump proposed a $1 trillion infrastructure spending program during his presidential campaign. The excitement, which has fizzled since, could soon be back, what with Trump finally planning to unveil his ambitious infrastructure plans in January 2018.

If Trump can, indeed, jump-start infrastructure spending in coming months, stocks from the sector could get a huge fillip and give investors a solid opportunity to play the upturn. The question is: Which stocks could benefit the most from Trump's $1 trillion infrastructure program? Remember, infrastructure isn't only about roads and bridges. It's a huge sector with at least 16 subsectors, and includes dams, railway tracks, waterworks, drainage systems, power and gas grids, transit systems, and pipelines, among others.

That also means finding good infrastructure stocks to buy can seem like a daunting task, given the options out there. Some examples include:

Building materials like aggregates and cement supplier stocks such as Vulcan Materials and U.S. Concrete .

and . Building product stocks like Masco and Builders FirstSource that are into roofing, flooring, plumbing, and so on.

and that are into roofing, flooring, plumbing, and so on. Heavy construction-equipment manufacturers and rentals like Caterpillar NYSE:CAT) United Rentals .

. Essential raw material suppliers like steel stocks, Nucor NYSE:NUE) United States Steel Corporation NYSE:X)

Infrastructure and engineering services companies such as Aecom NYSE:ACM) Chicago Bridge & Iron, and Jacobs Engineering.

To help you make informed investment decisions, I studied several of these infrastructure companies in-depth and have come up with the top three infrastructure stocks that could benefit hugely from Trump's spending program, as and when it unfurls.

Top infrastructure steel stock: Nucor

Without steel, it would be difficult to construct bridges, pipelines, tunnels, and rail tracks, which is why steel stocks like Nucor and United States Steel are bound to be among the biggest beneficiaries when infrastructure spending takes off. Both industry stalwarts have seen their sales and profits grow in recent quarters and are on track to end fiscal 2017 on an encouraging note, thanks to strong demand from key end-user markets, especially nonresidential construction, automotive, and energy.

Between Nucor and United States Steel, I like Nucor better for three reasons: strong profitability record even during a downturn, a diversified portfolio, and an incredible dividend streak. Nucor's judicious investments on growth in recent years are now generating strong returns on invested capital. Earlier this month, the steelmaker raised its dividend for the 45th consecutive year, proving its mettle despite operating in a highly cyclical industry.

During the nine months through Sept. 30, 2017, Nucor operated its steel mills at 86% capacity compared to 80% in the year-ago period, indicating that end markets are already gathering steam. That's only going to pick up pace once infrastructure projects start up in the U.S. Moreover, Trump's strong stand against cheap and illegal steel imports could boost domestic demand, fueling Nucor's growth even more.

Top infrastructure machinery stock: Caterpillar

I recently discussed three stocks in the construction-equipment industry that could win big on an infrastructure upturn. But if you were to choose one, Caterpillar remains the top choice for two reasons: It is the undisputed global leader in construction machinery and has the broadest product portfolio among peers.

As I mentioned earlier, infrastructure goes beyond roads and bridges. From that standpoint, Caterpillar enjoys a huge competitive advantage, thanks to its dominant position as an equipment supplier to key sectors like mining, oil and gas, power, and transportation sectors. Why, Trump even advocated Caterpillar during his presidential campaigns, hinting that the construction giant could bag a big piece of government infrastructure projects once they kick off.

Caterpillar's incredible run-up in the past one year makes the stock look like it's already running ahead of itself; but from a business activity standpoint, Caterpillar's real turnaround is ahead, not behind. Analysts expect the company to grow its earnings per share (EPS) by almost 40% in the next five years. Because earnings will grow from a low current base, analysts' estimates may hit the nail if Trump's infrastructure spending plan takes off next year.

Top infrastructure services stock: Aecom

Aecom is incredibly well poised to benefit from Trump's infrastructure program, for the simple reason that it specializes in providing architectural and engineering services, and building and operating infrastructure assets, for government agencies. In fiscal 2017, nearly 22% and 15% of Aecom's revenue worth $18 billion came from the U.S. federal government and the U.S. state and local governments, respectively.

FY 2017 was a record year for Aecom, with its revenue hitting an all-time high of $18.2 billion and backlog growing 11% to hit a record of $47.5 billion. While its 2014 URS acquisition is providing a major thrust to Aecom's top line, the company is confident of growing its organic revenue annually at 5% between 2018 and 2022. At the same time, Aecom expects to grow its adjusted EPS at 12%-15% and generate cumulative free cash flow worth more than $3.5 billion during the five-year period.

Critics may consider Aecom's goals as overly ambitious, but I don't see why the company shouldn't be able to deliver if infrastructure spending picks up in the U.S. In fact, Aecom is among the handful of companies that consistently generated greater FCF than net income in the past five years, which, I believe, is a sign of management efficiency and financial fortitude. In short, Aecom looks very compelling right now as an infrastructure play from every aspect, be it business, operational performance, or financial standing.

The Foolish bottom line

With the tax reform bill already on the table, an infrastructure spending proposal is next on Trump's agenda. While projects may still take time to start, infrastructure companies like the ones discussed above are already getting busier, thanks to rising nonresidential construction spending. To add to the list of infrastructure stocks discussed here, you should also take look at a recent article where I picked my top three building-material stocks that are primed for strong returns once infrastructure spending picks up. Before you know, these stocks could get too hot to grab once Trump gets serious about rebuilding America's crumbling infrastructure.