Electric trains are still due to start running between Cardiff and London in 2019, a minister has said, despite the deferring of work on other lines.
Work on four routes serving Bristol, Oxford and Berkshire is now on hold.
Rail minister Paul Maynard said it would free up money to "deliver additional benefits to passengers" and added Wales would not suffer delays.
"My focus is to ensure that we deliver electrification to Cardiff on time," he said of the £2.8bn modernisation plan.
Responding to a question from Cardiff West MP Kevin Brennan during a Commons rail debate, Mr Maynard said he wanted to make sure "that passengers in south Wales benefit from that electrification to the fullest degree possible".
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