The train line between Abergavenny and Hereford is blocked because of a train hitting a tree, according to Transport for Wales.
Passengers said they have been stuck on the train for more than two hours.
They were informed that the train they were travelling on between Cwmbran and Abergavenny "clipped a tree" which scraped the sides and they were then told the engine would not start.
On Twitter, passenger Holly Morgan said she was on the train behind, which then stopped for several hours.
Another passenger said some people have been standing for many hours and the air conditioning has been off since the engine failed, with the temperature feeling "like 30 degrees".
Transport for Wales said a rescue train is on site and replacement buses have been arranged to run from Hereford as far as Shrewsbury.