NEW YORK — Amtrak says its $168 million operating loss this year was the smallest since 1973, its second full year in business. The government-owned railroad said Thursday it took in a record $3.4 billion in revenue in the 2018 fiscal year. Amtrak has been shrinking its annual operating losses in recent years and says it plans to eliminate them by 2021. Amtrak said ridership remained steady at 31.7 million passenger trips. It blamed the lack of growth on service disruptions such as a second summer of repairs at New York’s Penn Station. The railroad received about $1.9 billion in federal subsidies in the 2018 fiscal year. It invested about $1.5 billion in capital improvements. Also Thursday, Amtrak’s board authorized the purchase of new locomotives for service outside the Northeast Corridor.