Train drivers on the newly-built South West Rail Link have been issued with an urgent message: slow down.

The speed restriction has been slapped on the rail line just six months after the much-anticipated extension was completed.

Sydney Trains ordered drivers to effectively crawl along the track between Edmonson Park and Leppington at just 10km/h after cracks in the rails were noticed by engineers conducting a regular sweep.

Trains travelling in the opposite direction have been ordered to keep under 40km/h as part of the restrictions.

The $1.6 billion piece of infrastructure was delivered 12 months ahead of schedule and $300 million below budget.

Sydney Trains CEO Howard Collins said passenger safety was paramount.

“When our engineers travelled along the line doing inspection, they noticed cracks – so as a precaution we slowed the trains down,” he said.

“We put safety first.”

Trains are expected to be back up to normal speed by the end of the month.