UPDATED 05/19/12 5:01 p.m.

CHICAGO (CBS) — If you ride Metra this weekend, get ready for a few more NATO-related headaches.

As CBS 2’s Susanna Song reports, security measures that were announced some time ago are now in effect for the commuter train service across the board.

Early Saturday morning, passengers boarding the trains were met by security guards. Searches and screenings will be intense, and several items are forbidden.

LISTEN: WBBM Newsradio’s Bob Roberts reports

Bags may not be larger than 15 inches by 4 inches – smaller even than the average purse – and backpacks and luggage are prohibited. Food and beverages are also forbidden on Metra trains during the summit period, even if they are sealed.

In addition, no tools or pepper spray are permitted on the trains.

Commuters are reminded that forbidden items can be confiscated.

At the Oak Park Metra station, one woman learned about the restrictions on bag sizes the hard way. She tried to get on with a backpack, and security officers stopped her and shook their head “no.”

There was no time for convincing. The woman had to leave the train.

“I told them check it. Please check it. They don’t want to check it. They don’t want to go through the trouble,” the woman said. “Unbelievable – harassing the citizens for the NATO Summit.”

But the ban on backpacks was announced several days ago, and Metra reminded passengers over the loudspeakers.

Another commuter, Dan O’Brien, was well aware of the restrictions. He purposely brought just one small bag, and he quickly scarfed down his muffin so as not to be subjected to the ban on food.

“I think it’s more cautious than necessary, but I think it’s better to err on the side of caution,” O’Brien said.

A CBS 2 colleague caught security guards being a lot more lenient on a Metra train to Naperville, where two women clearly got onto the Metra BNSF Line with backpacks.

Also, we found a man with a backpack walk right past a railroad officer and still get on a train.

Meanwhile, the stations at the Museum Campus, 18th Street, McCormick Place, 27th Street and 47th Street/Kenwood on the Electric District are closed through Monday night.

The big service changes occur Monday, when 19 stations on the Electric District will close, including the entire Blue Island branch, although Metra Deputy Executive Director George Hardwidge said security will be tighter no matter what line you ride the next few days.

“With the resources we have available, we can accommodate what amounts to a full ridership and a full train schedule on Sunday,” Hardwidge told WBBM Newsradio’s Bob Roberts. “On Monday the ridership is much higher, there are many more trains, and the train schedule is much more complex.”

On Monday, the five stations closest to McCormick Place will remain closed, along with the entire Blue Island branch, four stations on the South Chicago branch, and stations between 63rd and 11th Streets on the Electric District main line — with the exception of 95th Street.

Metra announced Tuesday that the resources had been found to keep the 95th Street station open on Monday.

In addition, no trains scheduled to arrive inbound at Millennium Station terminal between 12:57 p.m. and 6:46 p.m. will operate on Monday. Outbound service on the Electric District Monday is expected to be normal all day, except on the Blue Island branch.