The increase was the 25th consecutive monthly rise in ridership, Caltrain said.
Part of the ridership increase is attributed to a rise in riders attending San Francisco Giants baseball games at AT&T Park, up 11% in August from the comparable month a year ago, Caltrain said.
Farebox revenue in August was up 12.1%, according to a staff report.
The ridership increases have bolstered Caltrain's political and fiscal position at it gets set to advance electrification of its operations. California last week released $40 million last week in Proposition 1A bond money to begin work on the project, projected to cost about $1.5 billion.
Caltrain notes its Modernization Program is part of the "blended program" to provide both regional passenger service and the state's high speed rail network, which will share right-of-way in the Bay Area. The state's $700 billion project is expected to begin construction next year, after the state legislature reaffirmed its support last summer.