Passengers walk on underground metro tracks to be evacuated after an explosion at Maelbeek train station in Brussels, Belgium, March 22, 2016, in this handout courtesy of @OSOSXE via twitter. REUTERS/Courtesy @OSOSXE via Twitter/Handout via Reuters ATTENTION EDITORS – THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THE AUTHENTICITY, CONTENT, LOCATION OR DATE OF THIS IMAGE. FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS. FOR EDITORIAL USE ONLY. NO RESALES. NO ARCHIVE. THIS PICTURE WAS PROCESSED BY REUTERS TO ENHANCE QUALITY. AN UNPROCESSED VERSION WILL BE PROVIDED SEPARATELY.