Police in Canton Aargau said the attack did not appear to be a robbery and no personal relationship between the victim and the two men has been established.
“An act of racism is quite possible," spokesman Roland Pfister was quoted as saying by the 20 Minuten newspaper.
The victim received first aid at a nearby military facility and was later taken by ambulance to the hospital. The two attackers are being sought by the police.
The Eritrean community in Zurich has condemned the attack. According to Svenja Witzig, project manager at the Centre for Intercultural Conflicts, many people with dark skin refrain from using public transport in Switzerland, like train stations, for fear of attacks, 20 Minuten reported.