The girls, aged seven and eight, were reported missing from Balloch, near Loch Lomond, at about 8am – but were later tracked down on a train at Partick thanks to a quick-thinking commuter.
As the frantic call was made to police to report the girls missing, British Transport Police were being alerted by a concerned passenger on the 8.08am Dalmuir-Partick service about two girls travelling alone.
The woman asked the youngsters why they were on the train by themselves and they said they were going to visit their mother in Lanarkshire.
As the train pulled into Partick station at 8.25am, five British Transport Police officers got on and searched each carriage until they found the girls.
A BTP spokesman said: "We spoke to the girls, who are now in the care of Strathclyde Police. The passenger made the right decision in telephoning us."
A police spokeswoman said: "At around the same time as BTP was alerted, the girls were reported missing from an address in Balloch. They are safe and well."