Many services have been cancelled or replaced by buses and others are running less frequently.
The action follows the breakdown of talks with ScotRail on 6 July. Railway workers were also striking on Sunday.
The union is backing industrial action over concerns about the operation of more trains without guards.
On driver-only-operation trains, the driver is asked to close the doors at stations, instead of a conductor.
Such operations are already common on the ScotRail network, particularly on suburban rail lines.
ScotRail has said it would ensure a second member of staff was scheduled to be on board every train to assist the driver in emergencies.
However, the company has accepted there are occasions when a ticket examiner does not turn up for work that a train could run with only a driver and Scotrail has said it has union agreement for this.
The union believes the absence of a dedicated guard would make trains less safe.
Passengers are advised to check the ScotRail strike information web page before they travel.
The website lists more than 20 services that have been cancelled or have replacement bus services right across the country.
Several other routes are running with a reduced frequency.
The RMT is planning further strikes on Thursday, Saturday and Sunday.
Commuters are facing disruption to rail services as members of the RMT union enter their second day of strike action.