The Office of Rail and Road has said of driver-only operation: "Our inspectors are satisfied that with suitable equipment, proper procedures and competent staff in place, it is a safe method of working".
Sharon Keith, Northern's regional director, said the company was prepared to guarantee jobs and pay for conductors for the next eight years if an agreement could be reached on "better customer service".
Aslef, the drivers' union, confirmed its members had voted 4-1 to end its industrial action.
The letter said: "I remain concerned that in the event of a train evacuation, derailment or incapacitated driver, the absence of a guard to assist could leave passengers at risk". Commuters have to decide whether they want to take their chances with the reduced services promised by the train companies.More news: Scottish Parliament offices evacuated following discovery of 'suspect packages'
Members of the Rail, Maritime and Transport union (RMT) on Southern, South Western Railway (SWR) and Greater Anglia are due to strike today and Thursday while a 24-hour walkout will be held at Merseyrail and Arriva Rail North later today.
"We now look forward to working with Southern Rail to restore good industrial relations and deliver the service passengers in the region deserve". This is because we know that a second member of staff provides assurance to our passengers and ensures we have capable colleagues on board to deliver assistance or offer advice during journeys.
Hundreds of services will be canceled, replacement buses will be laid on and services that do run are expected to be busier than usual, passengers have already been warned.
A Transport Department spokesman said: 'The RMT is attempting to disrupt passengers as part of its political game.
"Driver-only operation has been operating safely for 32 years and now accounts for over a third of the United Kingdom rail network".
"It's also not about safety, as the independent rail regulator has ruled that driver-controlled trains which have been used in this country for thirty years, are safe".