The nation’s second-busiest rail line underwent a major milestone Monday morning when a new bridge opened over the San Diego River, allowing for more efficient travel.

SANDAG’s San Diego River Double Track Project was completed following four years of construction. The $93.9 million project allows trains among the Los Angeles-San Diego-San Luis Obispo corridor to cross at the same time.

“It’s the first time that both freight and passenger rail is going to be able to cross the San Diego River at the same time with trains being able to go in both directions,” said Ramon Ruelas, Mid-Coast Corridor Director of SANDAG.

The first train to travel through the newly completed bridge was Coaster 630, which was heading south from Oceanside.

“Trains won’t ever have to wait until one train crosses over the river before the next train goes through this area, so that’s a really big deal for this,” Ruelas told NBC 7.