BALTIMORE — Maryland’s congressional delegation wants CSX Transportation to explain why it will no longer back a train tunnel project that city and state officials had hoped would bring more cargo to the Port of Baltimore.

CSX said in a statement this week that it does not support expanding the century-old Howard Street Tunnel under downtown to accommodate trains with shipping containers stacked two-high. Balitmore’s port has seen increased shipments of the truck-size containers since the expanded Panama Canal opened last year.

In a letter to the CEO of CSX on Wednesday, congressional lawmakers said they want to know why a project that had been a priority for the railroad company — as well as local, state and federal officials — is suddenly no longer important to CSX, which owns the tunnel.