First Great Western has increased the number of seats available on its trains in the Thames Valley following a £29m investment in 48 additional carriages.
The 4,500 extra seats have been provided for customers heading to London Paddington from Oxford, Didcot and other stations to the west.
The additional seats arrived last Monday after 15 old buffet cars and three high-speed train carriages were converted.
All high-speed trains heading to the capital now have eight carriages instead of seven, providing about 85 extra seats per train.
First Great Western managing director Mark Hopwood said: “From the beginning of September all peak-time services have been strengthened, ensuring increased capacity for our passengers.
“This Department for Transport deal has helped provide the extra carriages that our customers have been asking for,” he added.