Service operator Transport for Wales says there is a fault with the signalling system between the two Powys towns and trains have to run on reduced speed on some lines.
A service affected today was the 6:22am from Birmingham New Street to Aberystwyth via Shrewsbury which was delayed at Newtown Powys and was running13 minutes late.
A spokesman TfW said: "Due to a fault with the signalling system between Welshpool and Newtown Powys trains have to run at reduced speed on some lines."
They say the impact of this will be train services running through these stations may be delayed.
This disruption to services is expected to run until the end of the day.
The company's app also says customers can expect "major delays" to services on services out of Cardiff. These include the Cardiff to Shrewsbury route.
There are also "major delays" on the Shrewsbury to Chester, and Shrewsbury to Crewe/Manchester lines.
The company has been hit by the need to repair its Class 175 trains and the issue is still ongoing, the company says.
Jan Chaudhry-Van der Velde, chief operations officer at Transport for Wales, said: “We’d like to thank our customers for their continued patience as we continue to work to resolve the issues with our Class 175 trains.
“Our Class 175 trains have been receiving urgent safety checks and engine repairs, after a number of recent incidents.
"This has left us with a temporary shortage of rolling stock across the network and has meant we have had to redistribute rolling stock to affect the minimum number of passengers possible.
“As the rolling stock shortages have eased, we have been able to restore much of our timetabled service.
"However, some services are still being operated by different trains with less capacity than usual, so we’re continuing to work through the repair programme on the Class 175 trains.
"As soon as each train is repaired and passes the safety checks, it is brought back into passenger service.”