From the Metra Website: (translation...just stay home and cower)

Safety measures affecting passengers during the summit
Metra has implemented a wide range of safety measures during the NATO Summit. Passengers may see an increased law enforcement presence and K-9 units.

In addition, the following safety measures apply to riders of all Metra lines during the three days (May 19, 20 and 21) of the summit:

Riders may be subject to search and/or screening before boarding or while en route.
Riders may carry only one bag not exceeding 15 inches square and 4 inches deep. Boxes, parcels, luggage, backpacks and bicycles will not be allowed on trains. Banned items cannot be stored at Metra stations. They must be removed or they will be disposed of.
Riders may not carry any food on the trains. Liquids and personal effects (such as makeup) must be less than three ounces in size. This includes coffee and other beverages. Breast milk can be carried but is subject to inspection and should be declared during any screening.
Riders may not carry any type of tools, pipes, stakes, wood or weapons, including pocket knives and pepper spray, on the trains.
Law enforcement personnel must identify themselves and present their credentials and any weapons. Armed security guards will need to provide proper credentials and be subject to verification prior to boarding.
Failure to comply with these safety measures or instructions from law enforcement personnel, or attempted avoidance of screening, will result in ejection from the station or further police action.