The journey time between parts of south Wales and London will now be shorter as Great Western Railway launches its biggest timetable changes since 1976.
The network is promising faster journey times, more seats and more frequent services to key locations.
Monday morning saw disruption to train services running to and from Reading and London Paddington due to a signalling fault.
Customers were warned to expect cancellations and delays.
The changes come after infrastructure improvements have allowed services to run with a fully-electric timetable between Cardiff and London.
The Cardiff to London service will be shortened by 14 minutes to one hour 53 minutes.
The fastest journeys from Paddington to Swansea will be shortened to two hours 38 minutes.
extra seats between south Wales and London every weekday from May 2020.
Train timetables are changed twice a year, in May and December.
The botched change in May 2018 led to chaos, and passenger watchdog Transport Focus said travellers would be hoping for a smoother introduction with the latest changes.
Great Western Railway is also promising the continued roll-out of five-carriage Turbo trains to replace the current three-carriage Class 150 units on the Cardiff to Portsmouth route.
Stops on that service include Bristol Temple Meads, Bath Spa, Salisbury and Southampton Central.
The network has employed more than 350 extra on-board staff to accommodate the changes.
This timetable has been a number of years in the making, enabling us to deliver the benefits of new Intercity Express Trains, and modernised infrastructure, for our customers. With the changes we realise that investment in trains and infrastructure, and deliver on the promise we made to customers for more services, more seats and faster journeys.
Great Western Railway is urging customers to check train times before travelling.