About 100 trains chug each day along the Norfolk Southern bridge, across the mouth of the Cuyahoga River.

It’s the first obstacle boats must pass to reach the Flats from Lake Erie, the one that stops them often for a half-hour at a time.

Here’s why:

A bridge tender works at the site 24 hours a day, observing boat traffic and trains headed east or west toward the bridge, and contacting train dispatchers and operators.

Unlike other Cuyahoga bridges, which merely inconvenience cars when they rotate or lift, trains don’t stop so easily. Therefore, the bridge remains down whenever a train is barreling nearby.

The bridge is only 5-10 feet above the water line.

Hence, unless you’re in a kayak, the bridge is going to stop you.

And so the boats wait, floating or pacing back and forth across the river. They blare their horns. They radio the bridge operator on their marine radio, on channel 16.

“Norfolk Southern is committed to meeting bridge-opening requests in a reasonable amount of time,” Norfolk Southern spokesman Jonathan Glass wrote in an email. “The railroad works closely with local government authorities, commercial freighter interests, and recreational boaters to address the needs of the maritime community in Cleveland.”

It’s a common misconception that freighters and other commercial boat traffic have the right of way, said Coast Guard Lt. Cmdr. Marvin Kimmel.

The freighters may give the bridge tender more advance notice, which helps, Kimmel said.

The bridge was built in the 1940s and replaced an old, bulky swing bridge. It was designed by Howard, Needles, Tammen & Bergendoff – a Missouri company with roots that date more than 100 years – and cost $4 million.

Motors operate cables that raise and drop the span, according to a cleveland.com story. Counter weights act as balance, “similar to an elevator.” Forty ropes – 2 and 1/4-inch diameter each – raise and lower the cables.

All sorts of trains traverse the bridge. The other day there was a train loaded with dozens of John Deere tractors.