The train operator announced it was doubling the length of Sunday services and peak-hour Saturday services to four coaches on the new route after The Scotsman revealed some trains were so packed passengers were unable to board.
One evening peak weekday service will also be lengthened.
ScotRail Alliance managing director Phil Verster said yesterday that passenger numbers were being closely monitored to see which other services needed extra carriages.
However, he said these would need to be found from elsewhere on the network, although some could be taken out of maintenance.
Mr Verster said: “With rolling stock at a premium, we will have to very carefully consider which trains need extra vehicles.”
Campaign for Borders Rail chairman Simon Walton said: ‘I appreciate additional carriages cannot be pulled from a hat.
“The campaign does not countenance inconveniencing anyone unnecessarily for the sake of the Borders Railway
“However, we have been assured the first resort to deal with additional demand would be lengthened trains.”