Talk about a hot start.

The Dow Jones transportation average has been up each day in 2018, notching nine-straight days of gains for its longest winning streak in four years. And some market watchers see more gains to come in the short term.

Boris Schlossberg of BK Asset Management said the tax cuts will help transportation stocks, but only if the cuts result in higher consumer and corporate spending and a boost to U.S. GDP. These hopes have fueled the rally in transports — but, that's just the ideal scenario.

"It's basically an implied bet on the fact that GDP goes to 3 percent, perhaps even 4 percent, next year," Schlossberg told CNBC's "Trading Nation." "I'm dubious that we're going to get to 3 percent. That's why I think the rally could be cut short as we go forward."

The Phoenix Group's Max Wolff also sees short-term gains for Dow transports and is uncertain how long the rally might stretch.

Wolff said transports will continue to rally in the first quarter of 2018. However, by the second quarter, Wolff said, a combination of potential protectionist policies and rising commodities could become a headwind.

By the second quarter "at the absolute latest this turns into a sort of anvil around the growth's neck so we'd be real careful how long we stay in it," said Wolff.

Like forecasts by Schlossberg and Wolff, overall predictions for 2018 GDP are cautious. Economists surveyed by FactSet anticipate economic growth to rise to 2.5 percent in 2018 from an expected 2.3 percent in 2017. Growth should then ease to 2.2 percent in 2019 and 2 percent in 2020, according to a median estimate.

But for now, the rally in the Dow transportation index is a freight train. The index has risen 7 percent this year, running ahead of the 's 3.6 percent gain so far this year.

All but one of the index's 20 constituents are in the green, with United Continental, United Parcel Service and American Airlines at the top of the list. Southwest Airlines, the only component to post a year-to-date loss (though, just a small one), has been left out of an overall airline rally following a year in which it posted gains at least double its peers.

Earnings season could put an end to the Dow transports' winning stretch. Just two components — FedEx and Delta — have reported on their fourth-quarter so far. Three components — CSX, J.B. Hunt Transports and Kansas City Southern — are due to report this week.