Boris Johnson is calling on South West Trains to drop a plan to close ticket offices at weekends.
The Mayor will write to bosses at London's biggest rail operator, urging them to think again.
South West Trains has been accused of putting profits before passengers after proposing to cut staffing levels at 114 stations. A spokesman for the Mayor said: "He does not have direct control over management of rail services and this includes the reduction of SWT staffing or closure of ticket offices, which he would oppose.
"However, he intends to hold a summit with the train operating companies later this year where he will outline his expectations of them. The Mayor will write to SWT regarding these issues."
The Mayor made his promise after a question from Caroline Pidgeon, a Liberal Democrat member of the London Assembly.
She welcomed his pledge but said she would watch carefully to see he acts on it. A South West Trains spokeswoman said: "We will read the letter with interest and respond accordingly."
Transport for London recently dropped its plan to withdraw staff from 40 Tube ticket offices after travellers and workers protested.
The Evening Standard has been campaigning for better station safety. Key demands are more uniformed staff and improved CCTV.
