Rail services between Cardiff and Newport will be cut for two months because of upgrade work.
Between 19 August and 3 September, trains will be reduced with buses replacing trains over the bank holiday.
Then from 23 September to 29 October, a second phase of work to prepare the line for electrification will see services reduced again.
During this period, the Severn Tunnel will also close on weekends for maintenance with trains diverted.
Passengers were urged to check timetables before travelling ahead of a drop-in event at Cardiff's Radisson Blu hotel on Monday between 15:00 and 19:00 BST.
Electric trains are set to start running between Cardiff and London in the autumn.
Network Rail's Andy Thomas said: "We recognise that passengers will face some disruption, and we understand how frustrating this can be but this work is essential in preparing Cardiff for new electric trains."