UPDATED 05/21/12 7:13 a.m.

CHICAGO (CBS) — While commuters riding the rails will face increased security Monday morning, some riders may not have any trains at all to catch.

As CBS 2’s Roseanne Tellez reports, the first train left as scheduled at 5:08 a.m. from the Hegewisch station on the city’s Southeast Side. The changes on the South Shore and Metra systems actually went into effect over the weekend, but the situation is more challenging Monday with commuters heading back to work.

LISTEN: WBBM Newsradio’s Bernie Tafoya reports

On the South Shore Line, trains are being swept by bomb-sniffing dogs. All passengers are being removed from each train as it crosses over the Indiana state line and arrives at the Hegewisch station, while the dogs proceed with the sweep and passengers board an empty waiting train that has already been screened.

Passengers will be searched upon boarding.

On the South Shore Line, the 63rd Street and Museum Campus/11th Street stations are closed altogether. Also, all inbound trains arriving at Millennium Station on the South Shore Line are canceled from 11 a.m. to 7 p.m. Monday.

For the Metra Electric Line, which runs on the same tracks through much of the city’s South Side, inbound trains arriving at Millennium Station until noon will operate as usual, but no trains will be running on the Blue Island branch.

All inbound Metra Electric trains scheduled between to arrive at Millennium Station between 12:57 p.m. at 6:46 p.m. are canceled.

In addition, on the Metra Electric main line, trains will not stop at 111th Street/Pullman, 107th Street/Rosemoor, 91st Street/Chesterfield, 87th Street/Woodruff, 83rd Street/Avalon Park/79th Street/Chatham, 75th Street/Grand Crossing, 63rd Street, 47th Street/Kenwood, 27th Street, McCormick Place, 18th Street, and Museum Campus/11th Street.

On the South Chicago branch, trains will not stop at 87th Street, Cheltenham/79th Street, Windsor Park, and Bryn Mawr at 71st and Jeffery Boulevard.

The South Shore and Metra Electric lines both run directly under McCormick Place, where the NATO Summit is being held.

Meanwhile, commuters are reminded that tight security restrictions remain in place.

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.

Commuter Barbara Lewis told WBBM Newsradio’s Bernie Tafoya Monday morning that she has no problem with the extra security measures.

“That’s fine, but I made sure I took everything I needed to work last week, so I don’t have anything but my keys and my phone,” Lewis said. “No bags at all I’m taking today.”