The European Commission set the deadline after the companies sought approval on Friday. The EU competition watchdog can approve the deal with or without concessions, or it can open a four-month investigation if it has serious concerns.
Siemens and Alstom announced the deal last September, potentially bringing the French TGV and German ICE high-speed trains as well as signalling and rail technology under one roof, and the companies have since been engaged in informal talks with the Commission.
While the deal is seen as a Franco-German industrial breakthrough for French President Emmanuel Macron, it has angered opposition politicians and trade unions worried about job losses.
(Reporting by Foo Yun Chee; Editing by David Goodman)