By Lewis Krauskopf

NEW YORK (Reuters) - Add this to worries about Wall Street: The index of planes, trains and trucking companies, considered an important stock barometer of the U.S. economy's health, is struggling.

The Dow Jones Transportation Average swooned 10.2% in May, a far steeper decline than that of the Dow Jones Industrial Average or the S&P 500. For analysts who closely watch the transports, this could be a sign of deeper market stress.

The transports were dealt another blow late on Thursday when U.S. President Donald Trump vowed to impose a tariff on all goods coming from Mexico until the flow of illegal immigrants across the southern border ceases.

In response, the Dow transport index dropped 1.9% on Friday, with the S&P 500 falling 1.3%. Rail operator Kansas City Southern, which derives a big portion of its revenue from Mexico, saw its shares tumble 4.5%.

The Dow transports started 2019 strong along with equities in general. But while the S&P 500 minted a new all-time high in April, the Dow transports have so far failed to reclaim their record peak set last September.

Now, with stocks also hit by concerns about a growing U.S.-China trade dispute, the Dow transport average is about 16% below its Sept. 14 all-time closing high.

"I see the performance of the transports really making it difficult for the broad market to sustain a big upward move," said Chuck Carlson, chief executive at Horizon Investment Services in Hammond, Indiana.

The transport stock weakness compounds other investor concerns, including that the U.S. trade war with China and other countries will harm the global economy and whether an expected pick-up in U.S. corporate profits will materialize later in the year.

The 20 stocks in the Dow transport index, including airlines such as Delta Air Lines, rail operators like Union Pacific Corp and package delivery companies FedEx Corp and United Parcel Service Inc, are "leading indicators of the economy," said Miller Tabak equity strategist Matt Maley.

Therefore their underperformance, according to Maley, is a "sign that growth in the U.S. economy was starting to slow even before the trade negotiations started breaking down."

This year, as the market rebounded from its late-2018 dive, the transport index closed at 11,098.99 on April 24, within about 4% of its all-time peak last year. But by this week the DJT had dropped below 10,000, a technical support level that Maley and other strategists have been watching.

For the third time in recent months, the Dow transports fell to their lowest level compared to the Dow industrials since 2012, according to Refinitiv data, another indication of the stocks' struggles.