The strike was cut from 24 hours to just over six after an emergency meeting with the city's mayor - to avoid disrupting a Radio Italia concert in Piazza del Duomo, according to public transport company ATM.
The walkout will now go ahead between 8.45am and 3pm.
“After 3pm, service is guaranteed for the rest of the day," ATM said.
Buses, trams and commuter trains could be impacted, though taxis were not expected to be involved in the strike.
Commuters were advised to plan ahead, check the status of their ATM service on the company's website, and to expect heavier traffic than usual on Milan's roads on Friday as a result of the strike.
As is usually the case with strikes in Italy, it wasn’t clear ahead of time how many staff would take part - and therefore how disruptive the strike would be.
Participation rates in other recent Al Cobas strikes have been around 12 percent.
The union said it was protesting the privatisation and subcontracting of ATM’s services and calling for higher pay and better contracts and safety conditions.
"The system only works because those who drive, repair, and clean are sacrificing their health and private lives," organisers told local news outlet Milano Today.
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