By Eric Sylvers in Milan and Takashi Mochizuki in Tokyo 

Hitachi Ltd., moving to expand its transportation business overseas, Tuesday signed a binding agreement to buy two units from Finmeccanica for about $1 billion as the Italian company looks to lower debt and concentrate on its core aerospace and defenses businesses.

Hitachi will pay EUR773 million ($876.1 million) for Finmeccanica's 40% stake in rail signaling operator Ansaldo STS and EUR36 million for unprofitable train manufacturer AnsaldoBreda, the two companies said in a statement. Following the closing of the deal, expected by the end of the year, Hitachi will launch a mandatory tender offer on Ansaldo STS's publicly traded shares which could boost the Japanese company's total payout to as much as $2.5 billion.

After restructuring to scale back its consumer electronics business, the Japanese conglomerate has been pushing to expand abroad, in areas ranging from train building to power-generation equipment to reduce its reliance on its slow-growing domestic market. The company moved the headquarters of its rail division to London from Tokyo last year after winning an order to supply high-speed trains in Britain.

For Finmeccanica, a third owned by the Italian government, the sale has been a long time coming, as the company continues to implement a plan to lower debt and concentrate resources on its aerospace and defense businesses.

Finmeccanica has been calling asset sales a priority since at least 2012, but potential transactions have often been met with national hand-wringing and demands that jobs be protected as part of any disposal deals.

Finmeccanica sold its power-plant construction business in late 2013 after more than 18 months of on-again off-again negotiations with various potential buyers. An offer by Siemens to buy the unit lead to a political backlash and later a rival offer from a fund backed by the Italian state eventually won out. Ansaldo STS has 4,000 employees, with about a third of them in Italy, while AnsaldoBreda has 2,300.

"We will keep the current employees and factories of AnsaldoBreda and we will utilize those of Ansaldo STS," said Hiroaki Nakanishi, chairman and chief executive of Hitachi. "As for the current management of the Italian companies, our first priority is to utilize the current structure."

The difficulty in selling industrial assets in Italy makes Tuesday's announced sale a particularly noteworthy victory for the year-old government of Matteo Renzi, the Italian prime minister who has made attracting foreign direct investment an important component of his plans to rekindle economic growth. Italy has one of the highest ratios of debt to gross domestic product in the world and is just now tentatively beginning to emerge from a triple-dip recession. The government and the International Monetary Fund are forecasting 0.6% growth this year.

Hitachi will pay EUR9.65 per Ansaldo STS share, a 9.2% premium to Monday's closing share price. The Italian company's shares have gained about 25% in the past 12 months.

"To be honest, the amount of cash we will be paying is larger than what I had initially expected because of the weak yen and also the Italian company's stock price moves," Mr. Nakanishi said.

Finmeccanica said the deal will lower its net debt by EUR600 million and lead to a capital gain of EUR250 million. Finmeccanica had negotiated with several other potential bidders and, for a time, a deal seemed close with a group led by Insigma Group of China.

Any dividends paid by Ansaldo STS before the closing of the deal will be deducted from the final price.

Ansaldo STS was 6.4% higher in midday trading in Milan at EUR9.40 while Finmeccanica, which has surged by more than 40% in the past three months, shed 2.9% to trade at EUR10.64.

Mediobanca and UBS advised Finmeccanica while Citigroup assisted Hitachi.

Manuela Mesco contributed to this article.

Write to Eric Sylvers at eric.sylvers@wsj.com and Takashi Mochizuki at takashi.mochizuki@wsj.com

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