The long-running row on Southern Rail revolves around a dispute over the role of guards on trains.
The train operator wants to give responsibility for closing carriage doors to the driver - but the unions claim this is inherently unsafe for passengers.
Although the dispute began with the RMT calling a series of strike dates, the involvement of ASLEF train drivers has brought widespread disruption to services in Kent, Sussex and Hampshire.
Both sides remain entrenched, each claiming the other is refusing to meet for talks to resolve the dispute.
Mick Whelan, ASLEF general secretary, says that he's been frustrated by attempts to have talks with Govia Thameslink Railways, the owners of Southern.
Angie Doll, Passenger Services Manager at Southern, said that striking was not the solution.
Meanwhile, the Transport Secretary Chris Grayling says the row has become politically motivated and is urging both sides to sit down for talks.