Amtrak and commuter rail service across the Susquehanna River was back to normal Thursday morning, hours after a tugboat crashed into the bridge between Harford and Cecil counties, causing delays to trains while inspectors checked the span and deemed it safe.

The incident tied up rail traffic on the busy Northeast Corridor line from about 10 to 11:20 p.m. Wednesday, Amtrak spokesman Craig Schultz said.

An operator from the tugboat called the crash in to the U.S. Coast Guard at about 9:30 p.m., Coast Guard Petty Officer Jonathan Lindberg said.

Four Amtrak intercity trains and one MARC commuter train were delayed while an engineer inspected the bridge and the two tracks atop the structure, Schultz said.

The bridge, which is owned by Amtrak, was not damaged by the boat, according to Schultz, who said did not have any more information on the boat or the circumstances of the crash.

No one was injured aboard the tugboat, which was towing a 295-foot barge, Lindberg said. He did not know how many people may have been on the boat or who owned it.

"I would not say it's an everyday occurrence, but it's not the first time that something like this has happened," Schultz said. "We have a procedure in place and we follow that procedure."

Lindberg likewise said he had not heard of any similar incidents recently.

The 108-year-old bridge between Harford and Cecil counties is being studied for a possible replacement. The steel deck bridge sits atop large block filings and has a swing truss draw.

Generally, the only tugboats working on the lower Susquehanna are used to haul barges of crushed rock from the Vulcan Materials quarry a few miles north of the bridge on the Harford County side of the river. The tugs and barges can navigate under the bridge's center span without the draw having to be opened.

Amtrak launched a three-year study recently to look at improvements to or possible replacement of the bridge, which it also wants to widen to accommodate at least four tracks.

"One of the things we are considering in that process is better navigation channels, or the impact on marine traffic," Schultz said.

He could not say, however, if a new bridge design could have prevented a crash like Wednesday's.

The study will consider a variety of options for better handling river traffic, such as raising the clearance so Amtrak does not have to open and close the bridge, Schultz added.

"It's very conceptual at this point," he said.

Sean Welsh of the Baltimore Sun contributed to this article.