By Mike DeNardo

PHILADELPHIA (CBS) - You may find less elbow room on SEPTA regional rail trains, as rail cars are taken out of service for federally required safety upgrades.

On a given day, seven to ten regional rail cars will be out of service, so SEPTA can meet a federal December 31st deadline to install "positive train control" on its entire 350-car fleet.

"It's going to impact riders," says SEPTA spokeswoman Jerri Williams.

PTC is a system designed to help trains avoid collisions like the AMTRAK derailment. Williams says typically, a six-car commuter train may only be five cars long:

"On many of our lines, we are filled to capacity. And so having one car missing could definitely result in overcrowding."

Williams says with normal inspections and hot-weather repairs, the fleet has been depleted at times:

"We have had some weeks recently where we have had drastic shortages. We think that we've got a handle on that now."

SEPTA on its website asks riders for their patience and understanding.