ASLEF is a trade union in the United Kingdom for people who drive trains. The letters stand for Associated Society of Locomotive Engineers and Firemen: “locomotive” is the engine that pulls the train, and “firemen” in the old name were the workers who once fed coal into steam engines, not the people who fight house fires. You see the name in British news when train drivers vote on pay or go on strike. It is a proper name, not a general English word for any rail union. Students outside Britain only need it if they are reading UK news. People often say it as a word, like “az-leff.”